Elle garde tous ses livres de son enfance dans une boîte.

In this sentence, garde (from garder) means to keep / to hold on to / to store.

• Elle garde tous ses livres...
  → She keeps all her books... (she still has them; she hasn’t thrown them away)

Why garder and not avoir?

• avoir = to have / to possess, more neutral:
  
  • Elle a tous ses livres de son enfance. = She has all her childhood books (just stating possession).
• garder adds the idea of keeping something deliberately, not getting rid of it:
  
  • She is purposely holding onto them, maybe for sentimental reasons.

Why not conserver?

• conserver is possible, but more formal or slightly more “careful / preserve” in tone:
  
  • Elle conserve tous ses livres de son enfance. (sounds a bit more formal or literary).

So garder here is natural, everyday French for “to keep” in the sense of not throwing away, storing.

The form of tout (all) must agree in gender (masculine/feminine) and number (singular/plural) with the noun it modifies.

• livre is masculine singular → un livre
• livres is masculine plural → des livres

So:

• masculine plural: tous ses livres
• feminine plural would be: toutes ses … (e.g. toutes ses maisons = all her houses).

Since livres is masculine plural, the correct form is tous, not toutes.

French possessive adjectives (mon, ma, mes / ton, ta, tes / son, sa, ses) agree with the thing possessed, not with the person who owns it.

Here, the owner is elle (she), so we choose from son, sa, ses.
Then we look at the noun:

• un livre → masculine singular
• des livres → plural

For a plural noun, regardless of gender:

• we use ses (his/her/its).

So:

• son livre = his/her/its book (1 book)
• ses livres = his/her/its books (more than 1 book)

That’s why it must be ses livres, not son livres.

You’re right: enfance is feminine.

Normally, for a feminine singular noun we would use sa:

• sa maison (her house)

However, French changes sa to son when the following word starts with a vowel sound, to make pronunciation smoother and avoid a clash of vowels.

• enfance starts with a vowel sound [ɑ̃]
• To avoid sa enfance (awkward to pronounce), French uses son enfance instead.

So:

• son enfance = her childhood (or his childhood), even though enfance is feminine.
  This is purely for euphony (sound), not because enfance is masculine.

Yes, in this sentence the -s of tous is silent.

General rule for tous:

1. Before a noun (adjective meaning all):

   • tous ses livres → pronounced [tu] (no s sound)

2. Used alone as a pronoun (meaning all of them / everyone):

   • Ils sont tous là. → pronounced [tus] (you hear the s).

So here, tous ses livres is pronounced approximately:

• [ɛl gaʁd tu se livʁ də sɔ̃n ɑ̃fɑ̃s dɑ̃zyn bwat]

In de son enfance, the preposition de expresses a relationship of origin or time:

• les livres de son enfance → literally the books of her childhood → more naturally: her childhood books / the books from her childhood.

Common English translations:

• de son enfance → from her childhood / of her childhood / from when she was a child

So de here is like “from” or “of” linking livres to a time period, son enfance.

Yes, Elle garde tous les livres de son enfance is grammatically correct.

Subtle nuance:

• tous ses livres de son enfance

  • Focuses on her books, the ones that belonged to her.
  • Suggests: all the books she had when she was a child.

• tous les livres de son enfance

  • Slightly more impersonal; it sounds more like all the books from her childhood as a period, not emphasizing ownership as strongly.
  • In context, it still normally means they were her books, but the possessive ses makes that clearer and more personal.

In everyday speech, both are very close in meaning; ses just makes the possession more explicit.

Elle garde is the present tense and, in this context, it describes:

• a current state or habit: she keeps them (she still has them, they are still in the box).

So:

• Elle garde tous ses livres de son enfance dans une boîte.
  → She keeps all her childhood books in a box (they are still there).

If you say Elle a gardé tous ses livres de son enfance, in passé composé:

• It emphasizes the completed action in the past of keeping them.
• Depending on context, it might suggest more that she did keep them up to some point, without necessarily highlighting the current situation.

To clearly express that she still has them now, the present elle garde is very natural.

Dans is the usual preposition for inside a physical container or enclosed space:

• dans une boîte = in / inside a box
• dans un tiroir = in a drawer
• dans un sac = in a bag

En is not used for that literal “inside a container” meaning.

• en is used more with materials, states, means, etc.:
  
  • en bois (made of wood)
  • en voiture (by car)
  • en vacances (on vacation)

So for “keeps … in a box”, dans une boîte is the correct and natural choice.

The article choice in French (indefinite vs definite) works similarly to English here.

• une boîte = a box (indefinite)

  • We’re not identifying any specific, known box; it’s just the container she uses.
  • Neutral, typical way to say this.

• la boîte = the box (definite)

  • Refers to a specific box known to the speaker and listener:
    
    • e.g. Elle garde tous ses livres de son enfance dans la boîte sous le lit.
      (the box under the bed that we both know about)

In your sentence, without extra context, une boîte is natural because the box has not been mentioned or identified before.

Yes, that word order is possible, and it’s grammatically correct:

• Elle garde tous ses livres de son enfance dans une boîte. (most neutral)
• Elle garde dans une boîte tous ses livres de son enfance.

The second version:

• Puts dans une boîte closer to the verb garde,
• Slightly emphasizes the location (“in a box”) more.

Both mean the same thing; the original order is the most common, neutral way to say it in everyday speech.