OK, I’m happy to get grief for this, but I did a sponsored famine every year for World Vision when I was a kid (well from age 10 to 18), they did a 40 hour famine (normally 8pm Friday to noon on Sunday) You were allowed to have either a spoon of honey or a boiled sweet once an hour, and drink as much water as you wanted.
For me, it was a great experience, not only did it make me feel like I was doing something to help a greater cause (all money went to charities to feed starving kids in Africa) it also made me realise what those kids have to go through on a daily basis and how lucky I was, to not have to worry about where my next meal was coming from.
As for the SU thing, again it’s something I was involved in as a kid through my school, unless you’re extremely anti religious I can’t see why you would not want to expose your child to this, it will broaden her horizons, give her a taste of religion and if it’s a path she wishes to chose then let her do it, if not it’s a harmless life experience, which in my experience introduced me to a number of lovely people a lot of whom I am still in contact with.