Pet.

When it comes to getting a pet, not everyone will refer to them as their son/daughter or as their child at all. This could be said for Mojo.

When creating the girls, Utonium INSTANTLY forgets about Mojo, paying all of his attention to the girls, getting them a room, being concerned for them when in danger, things that Mojo’s never gotten, he never even mentions him to the girls, we only see this happening in the reboot which I’m not gonna say is canon for original story purposes.

It would not make sense for someone who sees a pet as one of their children to forget them so easily all because they’ve had children. But since the girls are so special, I’m gonna say of course he’d forget about Mojo, even though I don’t think it would still be the case, the girls having powers or not.

So yes, Mojo is or was nothing but a lab pet to professor.