U vchodu do muzea jsme udělali fotku s maminkou.

Here u means at / by / near.

So u vchodu = at the entrance or by the entrance.

It does not mean in the entrance. If you wanted in the entrance/hallway, you would normally use a different wording.

In this sentence:

• u vchodu = at the entrance
• do muzea = to the museum

So together: at the entrance to the museum.

Because the preposition u requires the genitive case.

The dictionary form is vchod = entrance.
After u, it becomes:

• vchod → vchodu

So:

• u vchodu = at the entrance

This is a very common Czech pattern:

• u domu = by the house
• u školy = by the school
• u stolu = at the table

Because do also requires the genitive case.

The basic form is:

• muzeum = museum

After do, it becomes:

• muzea

So:

• do muzea = to the museum
• in this sentence, more naturally: the museum’s entrance / the entrance to the museum

So u vchodu do muzea is literally something like at the entrance of/to the museum.

Czech past tense is usually made from:

• a form of být = to be
• plus the -l participle of the main verb

Here:

• jsme = we are / we have as the past-tense auxiliary
• udělali = past participle of udělat

Together:

• jsme udělali = we did / we made / we took

So Czech past tense often looks like this:

• udělal jsem = I did
• udělali jsme = we did

In this sentence, the auxiliary comes before the participle because of normal Czech word order in this context.

Because jsme is a clitic, and Czech clitics usually go in the second position of the sentence or clause.

The first whole chunk here is:

• U vchodu do muzea

Then the clitic jsme comes next:

• U vchodu do muzea jsme udělali fotku s maminkou.

This is very normal Czech word order.

A different order is also possible:

• Udělali jsme fotku s maminkou u vchodu do muzea.

That version is also correct, but it gives a slightly different flow or emphasis.

Yes, literally udělat means to do / to make, but in Czech it is also commonly used in expressions like:

• udělat fotku = take a photo
• udělat si fotku = take a photo of oneself / get a photo

So udělali fotku is a natural colloquial way to say we took a photo.

Another common verb is:

• vyfotit = to photograph / take a photo

So you may also hear:

• Vyfotili jsme se s maminkou. = We took a photo with Mom / We got photographed with Mom.

Because in the Czech past tense, the participle agrees with the gender of the subject.

• udělali = used for a group that includes at least one male
• udělaly = used for an all-female group

So this sentence suggests that we includes at least one male.

For example:

• a son + his mother → udělali
• two women → udělaly

This is something English does not mark, so it often feels unusual to English-speaking learners.

Fotku is the accusative singular of fotka.

• fotka = photo, picture
• fotku = photo, picture as a direct object

Here it is the thing that was taken/made:

• udělali fotku = took a photo

Also, fotka is fairly informal and very common in everyday speech. A more formal word is:

• fotografie

Its accusative singular is:

• fotografii

So you could also say:

• udělali jsme fotografii
• but in everyday Czech, udělali jsme fotku sounds much more natural

Because the preposition s meaning with requires the instrumental case.

The basic form is:

• maminka = mom, mummy

In the instrumental singular, it becomes:

• maminkou

So:

• s maminkou = with Mom

This is the same pattern as:

• s kamarádkou = with a female friend
• s učitelkou = with a female teacher
• s ženou = with a woman

Maminka is a warm, familiar word, closer to mom, mum, or sometimes mummy depending on context.

It is not automatically childish. Adults often use maminka when speaking affectionately about their mother, especially in family contexts.

Some related words:

• matka = mother, more formal or biological
• máma = mom / mum, very common and casual
• maminka = affectionate, warm, often a little softer

So in this sentence, maminkou sounds natural and personal.

It most naturally suggests a photo with Mom — in other words, Mom is part of the situation and probably in the photo.

However, Czech can be a little loose here, just like English can be. Depending on context, it may mean:

• we took a photo with Mom
• we got a photo taken with Mom
• we have a photo together with Mom

If you want to make it very clear that we ourselves are in the picture with Mom, Czech often uses the reflexive form:

• Vyfotili jsme se s maminkou.

That is often the clearest way to say We took a photo of ourselves with Mom / We got photographed with Mom.

Because Czech word order is flexible, and speakers often put the setting first.

Starting with U vchodu do muzea gives the location first:

• At the entrance to the museum, we took a photo with Mom.

That sounds natural in Czech.

If you move things around, the meaning stays basically the same, but the emphasis changes:

• U vchodu do muzea jsme udělali fotku s maminkou.
  Focuses first on where it happened.

• S maminkou jsme udělali fotku u vchodu do muzea.
  Brings Mom more to the front.

• Udělali jsme fotku s maminkou u vchodu do muzea.
  Starts more directly with the action.

So the original sentence is completely natural and just sets the scene first.