Dust is very common and mistaken for orbs quite often. Also, how many children do you know that sit perfectly still and don't stir up dust?
If you really believe there are spirits in your pictures, that's okay, too. Let's just assume that's what it is. Great! You had some unseen visitors enjoying the child's party. If there are ghosts, who says they can't party, too? I really wouldn't worry about it too much.
The problem with orbs is this: If you capture an "orb" on camera, it can usually be explained. People have duplicated orbs with many different things. Moisture, dust, even bugs. Now, let's say you have a pic that has been looked at and scrutinized and found that the "orb" is genuine. That's fine, but the jury is still out as to whether orbs are spirits or just manifestations of energy (hmmmm, like children). This is why reputable ghost hunters don't claim a place is haunted when the only "evidence" they have is orb pics.
Hope that helps