Jak moje dcera, tak můj syn mají nejraději hřiště, kde je velká houpačka.

It is a paired expression meaning both my daughter and my son.

The pattern is:

jak X, tak Y = both X and Y

So:

Jak moje dcera, tak můj syn mají nejraději...
= Both my daughter and my son like ... best

This structure can sound a bit more formal or emphatic than simply using i:

Moje dcera i můj syn mají nejraději...

Because jak ..., tak ... is a correlative structure, and Czech normally separates its two parts with a comma.

So this is correct:

Jak moje dcera, tak můj syn...

There is also another comma later before kde, because kde je velká houpačka is a relative clause describing hřiště.

Because the subject is actually two people:

• moje dcera
• můj syn

Together they form a plural subject, so the verb must also be plural:

• má = he/she/it has
• mají = they have

So mají nejraději means they like best.

Because the possessive pronoun my must agree with the noun it describes.

• dcera is feminine singular, so: moje dcera
• syn is masculine singular, so: můj syn

This is normal Czech agreement. The pronoun changes form depending on gender, number, and case.

Literally, it is something like they have most gladly, but in normal Czech it means:

they like ... best
or
... is their favorite

The key word is nejraději, which expresses strongest preference.

Compare:

• rád / ráda = glad, likes
• raději = rather, more gladly
• nejraději = most gladly, best, most of all

So:

mají nejraději hřiště
= they like the playground best
or
= the playground is their favorite

Because it is the direct object of mají nejraději:

Mají nejraději co?
hřiště

That means it is in the accusative case. But hřiště is a neuter noun whose nominative and accusative singular are the same.

So:

• nominative: hřiště
• accusative: hřiště

Even though the case is accusative, the form does not visibly change.

It is a relative clause that describes hřiště.

So:

hřiště, kde je velká houpačka
= the playground where there is a big swing

The word kde means where, and it connects the description to hřiště.

You can think of it as answering: Which playground?
Answer: The one where there is a big swing.

Because the subject of that clause is singular:

velká houpačka = a big swing

So Czech uses singular je = is.

If there were multiple swings, it would be:

kde jsou velké houpačky
= where there are big swings

Because the adjective velká must agree with houpačka.

• houpačka is feminine singular
• so the adjective must also be feminine singular: velká

This is standard adjective-noun agreement in Czech.

Compare:

• velký syn would be masculine
• velká houpačka is feminine
• velké hřiště would be neuter

Both are possible.

• nejraději is the more standard/full form
• nejradši is a very common spoken or less formal variant

So in everyday conversation, many speakers would naturally say:

Jak moje dcera, tak můj syn mají nejradši hřiště, kde je velká houpačka.

The meaning is the same.

Yes. A very common simpler version would be:

Moje dcera i můj syn mají nejraději hřiště, kde je velká houpačka.

This means exactly the same thing, but it may feel more straightforward to learners because i is easier than the paired structure jak ..., tak ....

The original sentence is still fully correct; it just uses a slightly more emphatic structure.