Ο εγγονός και η εγγονή της τής έφεραν λουλούδια και ένα μικρό γλυκό.

Because they do two different jobs:

• της after η εγγονή means her in the sense of possession:
  ο εγγονός και η εγγονή της = her grandson and granddaughter
• τής before the verb means to her:
  τής έφεραν = they brought her

So the sentence contains both:

• her grandson and granddaughter
• brought her something

Greek often uses the genitive weak pronoun for both possession and indirect object.

The accent is mainly helpful here because two identical-looking forms appear right next to each other: της τής.

It helps the reader see that these are not the same word doing the same job:

• της = possessive her
• τής = indirect-object to her

In practice, this accent is often used to avoid confusion in exactly this kind of sentence.

In Greek, weak possessive pronouns like μου, σου, του, της often come after the noun:

• η κόρη μου = my daughter
• ο φίλος της = her friend

So:

• η εγγονή της = her granddaughter

This is very normal Greek word order for possession.

Here it applies to the whole pair:

• ο εγγονός και η εγγονή της = her grandson and granddaughter

Greek can place the possessive pronoun after the second noun in a coordinated phrase, and it naturally covers both nouns unless context suggests otherwise.

Because the two nouns have different grammatical genders:

• ο εγγονός is masculine = the grandson
• η εγγονή is feminine = the granddaughter

Greek articles must match the gender, number, and case of the noun.

Because the subject is two people:

• ο εγγονός και η εγγονή = the grandson and the granddaughter

A compound subject joined by και takes a plural verb, so:

• έφεραν = they brought

Έφεραν is the aorist (simple past) of φέρνω (to bring).

So:

• φέρνουν = they bring / are bringing
• έφεραν = they brought

In this sentence, it refers to a completed action in the past.

Because weak object pronouns in Greek usually go before a finite verb:

• μου είπε = he told me
• της έδωσα = I gave her
• τής έφεραν = they brought her

So τής is in its normal position as an indirect-object clitic.

Because Greek, like English, can use a bare plural noun when speaking indefinitely:

• έφεραν λουλούδια = they brought flowers

This does not mean specific flowers already known to the listener; it just means flowers in general.

If you said τα λουλούδια, that would sound more like the flowers.

Because γλυκό is a neuter singular noun, and both the article and adjective must agree with it:

• ένα = neuter singular a / one
• μικρό = neuter singular small
• γλυκό = neuter singular noun

So:

• ένα μικρό γλυκό = a small dessert / sweet

Here γλυκό means a sweet thing, usually a dessert or a sweet treat.

Depending on context, it could be translated as:

• a dessert
• a sweet
• a little cake / pastry (if the situation suggests that)

It does not necessarily mean the adjective sweet here; in this sentence it is a noun.

Because adjectives in Greek must agree with the noun they describe.

Since γλυκό is neuter singular, the adjective must also be neuter singular:

• masculine: μικρός
• feminine: μικρή
• neuter: μικρό

So:

• ένα μικρό γλυκό is correct

Because λουλούδι (flower) is a neuter noun, and its plural is:

• singular: το λουλούδι
• plural: τα λουλούδια

In the sentence, λουλούδια is the plural form meaning flowers.

Greek uses the definite article much more often than English does.

So Greek says:

• ο εγγονός και η εγγονή της

Even if natural English may simply say:

• her grandson and granddaughter

This is a very common difference between the two languages.

Yes. Greek word order is more flexible than English, as long as the grammar is clear.

For example, you could also have:

• Ο εγγονός και η εγγονή της τής έφεραν λουλούδια.
• Τής έφεραν λουλούδια και ένα μικρό γλυκό ο εγγονός και η εγγονή της.

The original order is natural, but Greek often moves parts around for emphasis or style.