1: does washing hands or face after wudu invalidate wudu??
2: helping a child in taharat invalidate wudu,, when najasa is on our hand???
3: during ghusl, is it compulsory/wajib to wipe aaza e ghusl with hand ((as in wuzu)) or just reaching water/washing is enough?

1- after the completion there is no harm in it.
2- only you need to do taharat of the hand wudhu is valid and in order
3-wiping is not must