The gods of my fantasy world have returned, but I can't figure out why.

The gods were a species who could use magic with far more strength than humans. They are immortal, with perpetual youth. They have immense physical strength and also unique magical abilities of their own. However, due to infighting and a rebellion from the humans they oppressed, they were driven to extinction.

Later, about seven centuries later, several children were born with special abilities that would lie dormant until they were physically mature. They were new gods.

They are not the same people as the old gods, but their abilities are the same as the old gods.

I'm not sure how the gods came around again. Any ideas? I was thinking that magical experimentation could be the case, but I'm not sure.

So, there are several possible reasons for the "return of the gods." You could go with something cliched, like "they sensed a new danger rising," or "they decided that it was their time to rule again," but honestly I think we can do better than that.
So, I present this theory: the "gods" as we know them aren't really different from regular people. Their immortality, and superpowers, and whatever else makes them special, comes from the spirits of the "gods." They are semi-sentient begins, driven by instinct, able to form symbiotic relationships with their human hosts. They choose hosts based on health, strength, beauty, etc… In exchange for keeping them safe, they grant their hosts extraordinary power and immortality, making them "gods" to the eyes of regular humans. When their first hosts were killed, the spirits scattered to the winds. It took them seven hundred years to gather enough strength to come back and find new human hosts.
An idea of a conflict could be that the gods have different ideas of what their hosts should be doing than their actual hosts. The gods are more primal, more animalistic, more driven to survive and thrive overall and this causes them to struggle with their hosts.
Hope it helps you!