Yeah this happens to me too. I'll be doing something, and get an erection, but it's not like I'm horny or anything. I could even be tickling a girl, just for fun, and have an erection while not being arroused. It CAN happen! There's different types of "erections" I think.
Type 1 is the normal one, we're completely horny and want to do it right then and there with whoever just made us horny. We can't even think straight because of how horny we are. This barely ever happens on its own - there's usually a girl involved in this case.
Type 2 is when it's up, and we're horny sort of, but we don't really feel like doing it. The difference between this, and the above case, is that no girl is involved. Like sometimes I'll be at work, and it'll just get hard for no reason, and I'll feel like... touching it. This happens randomly, and I'm sure it happens to girls too. Don't you sometimes feel like randomly getting laid all of a sudden?
Type 3 is when it goes up because of physical stimulation. You can be the most unattractive girl in the world, but if you tie a guy up and begin playing with his thing, it's gonna get hard, but it doesn't mean he's arroused.
And type 4 is when it goes up on its own, completely without reason. This can happen anywhere. It happened today when I got to work and sat down on my chair. I wasn't thinking sexual thoughts, and there were no hot girls around. However, I wasn't aroused. In fact, most of the time, I don't even notice this happening.
So I wouldn't worry if he was playing with your new baby and he happened to have an erection. It doesn't mean anything. And I seriously doubt she was jumping directly on top of his thing. That's like having a baby jumping directly on top of your boobs... I think. I mean, it's not like I would know. xD