Can a rusty chain be saved?
If the chain isn't worn (which it probably is not, kids bikes don't get high mileage) it might be worth trying to save, but you may end up spending more compared to getting a new one.

Penetrating oil, a wire brush and a lot of effort may free it up enough and get it reasonably clean.

Removing the rust chemically is problematic. Something strong enough to work with the chain in place may damage paint or plastics. There are safe products such as Vapo-Rust but the chain would need to be removed and soaked for an extended period of time. Also, new chains for 10 or 15 speed bikes are inexpensive. Instead of buying chemicals, just buy a chain.