Después de haberse secado, mi hija me dijo que la arena dentro del bañador era más incómoda de lo que imaginaba.

Because after después de, Spanish normally uses de + infinitive when it means after doing something.

So:

• después de haberse secado = after having dried herself / after drying off

It refers to an action that happened before the main one:

• first, she dried off
• then, she told me something

The subject is understood to be mi hija.

It is made of three parts:

• haber = auxiliary verb used to form perfect tenses
• se = reflexive pronoun
• secado = past participle of secar

So literally it means to have dried oneself.

In natural English, depending on context, it can be:

• to have dried herself
• to have dried off
• to have gotten dry

Here the reflexive idea is important: your daughter is the one who got dry.

Because Spanish object and reflexive pronouns can attach to an infinitive.

With a compound infinitive, the pronoun attaches to haber, not to the past participle:

• haberse secado
• haberte visto
• habernos ido

So haberse secado is the normal form, not anything attached to secado.

Yes.

All of these are possible:

• después de secarse
• después de haberse secado
• después de que se secó

The difference is mostly one of nuance:

• después de secarse = simpler and very common
• después de haberse secado = slightly more explicit about the action being completed beforehand
• después de que se secó = uses a full clause instead of an infinitive

In this sentence, después de haberse secado sounds natural and a bit more polished.

Because decir usually tells you who was told something.

• mi hija me dijo = my daughter told me
• mi hija dijo = my daughter said

So me is the indirect object pronoun meaning to me.

Because incómoda is describing la arena, not how a person felt.

• la arena era incómoda = the sand was uncomfortable
• mi hija estaba incómoda = my daughter felt uncomfortable

With things, ser incómodo/a is very common for meaning to be uncomfortable / inconvenient / awkward.

If you said la arena estaba incómoda, it would sound odd, almost as if the sand itself were feeling uncomfortable.

Because era presents the discomfort as a state or ongoing impression at that moment.

• era más incómoda sounds like a description or evaluation
• fue más incómoda sounds more like a completed event or summary judgment

In reported speech like this, era is the more natural choice.

Because the adjective agrees with la arena, which is feminine singular.

• la arena → incómoda
• el suelo → incómodo
• las piedras → incómodas

So the sentence is not saying that your daughter was uncomfortable. It is saying that the sand was uncomfortable.

This is a very common comparison pattern in Spanish:

• más + adjective + de lo que + verb

So:

• más incómoda de lo que imaginaba = more uncomfortable than ... imagined

Here lo que does not refer to a specific noun. It refers more generally to what or the degree to which someone imagined it.

Compare:

• más incómoda que la toalla = more uncomfortable than the towel
• más incómoda de lo que imaginaba = more uncomfortable than I/she imagined

A simple rule:

• use que before a noun or pronoun
• use de lo que before a clause with a verb

Yes, it can be.

In the imperfect, -ar verbs have the same form for I and he/she:

• yo imaginaba
• él/ella imaginaba

So de lo que imaginaba could mean:

• than I imagined
• than she imagined

Context usually resolves it. In this sentence, many readers will assume it means than she imagined, because your daughter is the one experiencing it. But grammatically, the form itself is ambiguous.

If you want to make it clear, you can say:

• de lo que ella imaginaba
• de lo que yo imaginaba

Dentro de means inside or inside of.

So:

• dentro del bañador = inside the swimsuit

And del is just the contraction of:

• de + el = del

So literally:

• dentro del bañador = inside of the swimsuit

This suggests the sand was trapped inside the garment, not just on it.

Yes. Bañador is very common in Spain for swimsuit or bathing suit.

In other Spanish-speaking regions, you may hear other words, such as:

• traje de baño
• vestido de baño
• malla

So this sentence sounds especially natural in Peninsular Spanish.