My sim was changing her toddlers nappy on the changing table, when for some reason she picked up the other toddler and now the toddler that was being changed is glitching and the game thinks hes a baby again and everytime i try to get his mum to age him back up by using sim blender or sim manipulator it throws any error that i dont understand. I have twice tried deleting him with moveobjects on but that does nothing.

I think that's a serious glitch or bug.
If hes a baby again maybe you can just play a few days and see if the game ages him like normal.

If a sim has de-aged to infant-in-older-body, they won't age up. They'll only glitch if someone tries to pick them up.
I think it's possible to change their age with SimPE.