>The parenting age adults can't exactly hunt and gather if they have to watch babies all day long- yet human children are pretty helpless and need almost constant care for years.

Gathering food or light manual work? You just stick 'em in a papoose (we used a shoulder sling). Heavier work, hang the papoose in a tree. Once they can walk they can gather food and do light work.

I'm not saying grandparents wouldn't make it easier, but babies/toddlers are quite adaptable.

I think for hunting (with weapons) you'd want them to be at least 5.