This isn't at all implausible: one of you should take 'agumonkey up on this! I wrote my first Scheme interpreter at 11, and an older friend of mine taught me Forth a bit before that. I wrote my interpreter with a bootleg (and ancient) APL2 copy, though.

With parental guidance, a 7 year old could totally do this. Figuring out how to handle the stack will be the trickiest part, and there are a few examples online of this, so if you nudge them in the direction of others' work, they should be able to get it pretty quickly. They'd finish it by the time they're 8, at least.