As far as I know part of the things that might invalid wudu' is touching oneself on the private parts.
As yesterday when i helped my little 3 years old son to sit on the toilet or a pisspot I needed to help him place his penis on the right position to not urinate in the bathroom instead. This question came into my mind:
Would such an act also invalid wudu'?
There is considerable debate whether touching your own private parts breaks wudu, the ruling heavily depends on madhhab. As for shafii fiqh, "reliance of the traveler" (unless I'm mistaken, this is the most respected legal manual in the shafii school) mentions that it does not matter whose private parts you touch, and whether you do it intentionally or not; as long as you touch with the inside of your hand, your wudu is broken.
I found a copy of the reliance as translated by Nuh Ha Mim Keller online, and on pages 72, 73 in section e7.4 it says word for word that which is quoted on the site I linked to. I don't know the legal status of linking to it here, so I won't.
As for other madhhabs, I don't know. I think the hanafis don't consider any touching of private parts to invalidate wudu by itself, and the hanbalis include not just touching with palms or inside of fingers, but any touching with hands. I don't know what the malikis say.
Disclaimer: I am not a muslim.
