The sentence you wrote is mostly good, but it can be made a little clearer with some small changes. Here is your sentence:
House martins and swifts find great nesting spots on top of tall buildings, where they feel safe.
It is grammatically correct! The sentence has:
- A subject: 'House martins and swifts'
- A verb: 'find'
- An object: 'great nesting spots'
- A prepositional phrase to say where: 'on top of tall buildings'
- A clause explaining why: 'where they feel safe'
So, no changes are needed. It's simple and clear, perfect for talking about birds' homes with a child.