I deleted a toddler sim who got stuck on the potty and he became a boddler. I went into SimPE and changed his age from Baby to Toddler, but he still couldn't click on the potty despite being potty trained and had only three days in his lifespan no matter how much I messed with his aging, so I aged him up to child using Sim Blender. He appears to be normal now, but I haven't thoroughly checked him. How much did I just mess up my game?

If he's fine now, I think you shouldn't have issues.

If he appears fine and now has the right age and looks, he should be fine.

If you're thinking about aging him down again, it's always a bit safer to age up than down. You can get some mess going on in wants/wishes and such when aging down, but it tends to be temporary (and isn't game/hood corruption, just a bit of an annoyance).