У ребёнка весь подбородок был в мороженом.

У + genitive is a very common Russian way to mean someone has... or to locate something on/with/about someone.

So:

• У ребёнка весь подбородок был в мороженом
  literally: At the child, the whole chin was in ice cream

Natural English: The child’s whole chin was covered in ice cream.

Russian often uses this structure when talking about body parts, possessions, or features:

• У него болит голова = His head hurts / He has a headache
• У девочки длинные волосы = The girl has long hair

So у ребёнка here means something like the child’s.

Because the preposition у requires the genitive.

The dictionary form is:

• ребёнок = child

But after у, it becomes:

• у ребёнка = of the child / by the child / the child’s

This is just standard case usage:

• у мамы
• у брата
• у ребёнка

So the form is not special to this sentence; it is required by у.

Because весь must agree with подбородок in gender, number, and case.

подбородок is:

• masculine
• singular
• nominative

So the correct form is:

• весь подбородок = the whole chin

Compare:

• весь нос = the whole nose
• вся щека = the whole cheek
• всё лицо = the whole face
• все руки = all the hands / both hands depending on context

Here весь emphasizes that the chin was covered completely, not just a little spot.

Because the verb agrees with the grammatical subject, which is подбородок.

In this sentence, the subject is:

• весь подбородок

And подбородок is masculine singular, so in the past tense the verb is:

• был

Compare:

• Подбородок был в мороженом = The chin was in ice cream
• Щека была в мороженом = The cheek was in ice cream
• Лицо было в мороженом = The face was in ice cream
• Губы были в мороженом = The lips were in ice cream

So the verb matches подбородок, not ребёнка.

Yes, literally it is was in ice cream, but idiomatically it means:

• was covered in ice cream
• had ice cream all over it

Russian often uses быть в + prepositional to describe someone or something as being covered, stained, or surrounded by something.

For example:

• Руки были в грязи = The hands were covered in dirt
• Рубашка была в крови = The shirt was covered in blood
• Он весь был в снегу = He was covered in snow

So подбородок был в мороженом does not mean the chin was physically inside a bowl of ice cream. It means ice cream was all over the chin.

Because after в in this meaning, Russian uses the prepositional case, not the instrumental.

Here:

• мороженое = ice cream
• prepositional singular: в мороженом

So:

• в мороженом = in ice cream / covered in ice cream

A learner may expect мороженым because the instrumental is often used for the material or substance causing a stain, especially with passive-style words:

• испачкан мороженым = stained/smeared with ice cream

So both patterns exist, but they are different constructions:

• подбородок был в мороженом = the chin was covered in ice cream
• подбородок был испачкан мороженым = the chin was smeared/stained with ice cream

Both are natural, but this sentence uses the в + prepositional pattern.

Because мороженое is historically and grammatically an adjective used as a noun.

Its base meaning is related to frozen, and in modern Russian it functions as the normal word for ice cream.

So it declines like an adjective:

• nominative: мороженое
• genitive: мороженого
• dative: мороженому
• prepositional: в мороженом

That is why в мороженом may look surprising if you were expecting a more typical noun ending.

The same thing happens with some other substantivized adjectives in Russian.

No, Russian word order is fairly flexible, and changing it usually changes emphasis, not the basic meaning.

The given sentence:

• У ребёнка весь подбородок был в мороженом

puts у ребёнка first, which sets the scene: as for the child...

Other possible orders:

• Весь подбородок у ребёнка был в мороженом
• У ребёнка подбородок был весь в мороженом
• Подбородок у ребёнка был весь в мороженом

These all sound a little different in focus, but the core meaning stays similar.

The original version sounds natural and descriptive, especially in narration.

Yes. In many Russian texts, ё is often written as е, so you may see:

• ребёнка
• ребенка

Both represent the same word.

However, the pronunciation is really yo:

• re-BYON-ka

Using ё is helpful for learners because it shows the stress and pronunciation more clearly.

So:

• ребёнок = correct with ё
• ребенок = very common spelling in ordinary print, but still pronounced ребёнок

Yes, absolutely. That is a very natural English translation.

Possible translations include:

• The child’s whole chin was covered in ice cream.
• The child had ice cream all over his/her chin.
• The child’s chin was completely smeared with ice cream.

The exact English wording depends on style, but the Russian suggests that a large area of the chin was messy with ice cream. The word весь especially supports translations like whole, all over, or completely.