I've had a few hits lately on people searching about how to baby/childproof a litter box.
Amazingly we haven't had any issues. Mateo is 16 months old, almost 17 months, a walker, a runner (sorta) and very inquisitive.
He has managed to learn that the litter box area and the cat food area are definite "no" zones. I think one of the things that helped us is we never made a big deal about the two areas, unless he does show some interest, and then we firmly tell him "No - that's the cats' food" or "No - that's dirty."
It also helps that he's never been left alone (too much) to wander around these areas. Granted, our condo is small, so he's usually in eyesight wherever he is.
With that said, I would suggest putting the litter box on a platform or somehow gating if off so the baby/toddler can't get to it. Those were our two thoughts, but then we realized how much of a non-issue it became.
I am still waiting for the day Mateo has a handful of litter or pee clumps in his hand. So far, so good.