Being a CNA since I was 18, I can definitely say I've been asked of too much. But it's all balance. There's been days I've ran around like a chicken with no head, and other days that I wonder if someone's playing a joke because it's so "quiet"."
I wouldnt worry that you're asking too much, because from my experience, I let others know when I can't accommodate them. "Yea I'll get that done, but right now it's at the bottom of my list." That's my nice way of saying it problem won't happen
At the same time, I've asked things of my nurse, very well knowing we're all busy. "Hey I need help pulling up this patient." "Mr. Smith is on the toilet, can you watch him while I go get clean socks?" Things like that.
If they're CNAs interested in furthering their career, definitely ask them along. I'd love when my nurses would ask me to help with wound care, glucose checks, IV starts, cath insertions, etc. I'd help the nurse by keeping the patient on their side or whatever, and as the nurse was doing their procedure, she/he would walk us step by step. I was always learning to help the nurses!