When you go wide angle, perspective is exaggerated. Things closer to the lens appear MUCH bigger than things even a little further away.
If you are shooting from above your kids, then their heads are closer to the lens than their bodies, so you get a distorted body image. Likewise, if you are pointing the camera downwards, all the upright lines in the room will distort drastically because of the same phenomenon of exagerrated perspective. You will notice this problem less with wide angle if you try and shoot very close to horizontal when there are obvious uprights to confuse the eye (things like walls, corners, chairs, tables etc).
The other possibility is that the lens has something called barrel distortion. This is where straight lines will look straight at the centre of the image, but bend outwards towards the edges of the image. This is a common problem with most wide angle lenses (some of the very expensive primes and maybe a good copy of the sigma 12-24 don't suffer badly). How straight the lines stay at the edges of the image is a measure of how rectilinear the lens is.
If you want a clear test to see if your lens is barreling: take a close-up photo of a coke can or something. Fill the frame with it. The trick is to get the straight lines of the can on or near the edge of your frame. If it looks like a barrel instead of a can, its got barrel distortion.
Move back, zoom in - it should go away as you use longer lens lengths.
I hope some of this helps a bit.