I think that has more to do with their capability as an overall fighter, not just power. The humans are all experienced enough to fight as individual units and not have to rely on somebody else to function at 100%, whereas the kids don't have that experience and have always been tag team fighters, more or less. Vegeta even pointed out that they've relied on fusion in every single battle they've had since its introduction, so they haven't learned to deal with not having that as their fallback, unlike the adult Saiyans, who have had fusion explicitly barred from use in desperate situations.
Can't speak for the anime too much, but luckily, that's not in the manga. Piccolo is given an automatic pass into the ToP as well by Whis, and there's no explanation given for the exclusion of the boys on Vegeta's part.