AND AND - i like to think that Swapfell doesn't actually experiment on kids, (specifcally kids under 13) theres a few reason why, this man conscious WOULD NOT let him, not at all, like he can try to defy his conscious (and his soul to an extent) all he wants, but it's not going to let him. (and if he doesn try, he'll just end up tired and confused) -- reason 2, he doesn't find human kids all that interesting, they cry and whine and whatever kids do, if he was still in the G.G house, and if you were like a 8 year old kid, he would probably ignore you for the most part. --
It's not like he doesn't like kids, it's just that it feels too familar to his own kids, like his mind would connect any personality from just one kid, and it'll unlock a forbidden memory. --
And if it was after the 'incident' and he magically sees this kid with one of the gaster's, he go like, "What the.. What is this.. Two year old doing in this limbo?" - Depending on the Gaster reaction, but it'll end up like "youre not going touch the kid.."
Swapfell will respond as, "Yeah, no I'm not planning too. It's already sad that this kid have to live their short lifespan in this desolate void. Anyways, I'm not interested with crybabies." And this man would leave, WAVING goodbye to the gaster he just encountered and continue on with his work +