この子ぐらいの服のサイズありますか？って英語でなんて言うの？

If you have your child with you then you could say 'Do you have this in my child's size?' as the shop assistant can see what size you need. You can also ask 'Do you have this in size......?' if you know what size you need. If you like an item and want to know what sizes they have in stock then you could say
'What sizes do you have this.... in please?'

1. Here you are asking the assistant if the shop has an item to fit your child.
2. Here you express that you want one of these items that will fit your child. you are asking indirectly if there is one available somewhere in the shop.
   A: ". I'm looking for one of these pairs of jeans to fit my child."
   B: " Certainly madam, I'll just check the stock room."

If you want to ask the staff at a clothing store if they have the size for your child, you can say:
"Do you have clothes in his/her size?"
"Do you have size ~ for kids?"

You can ask the question using the 'Do you have?' question form , followed by the specification of the size, e.g. ;small'

You can ask the staff if they have a specific size you want, or you can ask them what sizes they have available.

You can ask if they "stock" in a childs/kids size.

Do you stock clothing in his/her size. asks if they keep clothes in your childs size.

Do you have clothes for a 8 year old. You can replace the age with how old your child is.

So the best ways to ask a staff member at a clothing store would be to say "Do you have this in a size ..?" You would add the size in as well . You could also ask them to go and look in the back where the clothes are stored to see if they have your child's size.