Embarrassing a talmid, al pi Rambam, can be mutar, if it will motivate the talmid (Hilkhot Talmud Torah 4:5). Kids on this level won’t be motivated through embarassment, though.
The Az Nidberu (R’ Binyamin Zilber) 12:42 was asked if someone sleeping on their back or stomach ought to be woken up. He paskened that the issur is to go to sleep lying flat. Once a person is sleeping he is not over any issur and is not responsible to be in any specific position. Therefore you need not wake him up.
The issur of falling asleep perakdan comes from Berakhot 13b. The Gemara says that saying kriyat shema in this position is assur and R’ Yehoshua ben Levi curses one who sleeps like this. The Az Nidberu says that the peshat here is that it is assur to fall asleep face down or on the back.
You’re not endearing anybody to Torah by waking them out of their sleep.
Do you have someone who gives you eitzot on kiruv? If you’re doing this work, you need to have the proper rebbeim guiding you.