Nallid, does your kid's set-up have spacers? If not, that's why it gets slower, because you are aplying lateral pressure to the bearing, making unwanted side friction between the race and the balls. With spacers there's no pressure to the bearing so you can crack it all down and it will spin freely. This also prevents axle slip because there's no play for the axle to move when the nuts are cracked down.
So, I'm totally understanding what Roisto is saying.
(I dont use spacers tho, I never felt the need to)