Can I sign the skate park waiver for two children that aren't mine, can I be your guardian?

If you're over the age of 21 and pose as a DAD I don't see why not

Legally? No ... just from a liability standpoint you could be on the hook for any medical bills should something happen to the minors you signed the waiver for.

No, not legally. Getting away with that is a separate question...and probably yes.

I believe so yes