First communion ceremony w an R6

All right guys I do mostly outdoor family photography and I was just asked by one of my good friends who is a teacher to take photos at their first communion ceremony inside the church. It could turn into a pretty consistent gig with baptisms and other catholic ceremonies so I agreed no questions asked lol. I have a Canon R6. I have a 35, a 50 and 85 all f1.8 and the only zoom lens I have is a 75-300 f4. They don’t allow flash in ceremony, but I can use flash to take the group photo at the end if I need to.

Each child will receive one- two photos in ceremony of them approaching Priest and receiving communion and the group photo at the end with all participants.

What settings would you use? What lens would you use out of the four I have?

I’m obviously gonna have to crank my ISO up pretty high and I’m a little worried about grain. I just really wanna make sure I do my best.