Are there.........children in the park?(many, much, a)

Are there many children in the park?  Есть много деток в парке?

Many (много) is used with countable nouns.

Much (много) is used with uncountable nouns. 

A is indefinite article. Indefinite article is not used with plural nouns.

There are many books in the bookcase.

There is much water in the cup.

There is a book on the table. 

There are many animals on the farm. 

She drinks much coffee. 

There is a pear on the plate. 

He has got many lessons today. 

He has much time to do his lessons. 

He has a lesson today.