I don't have any specific apps to recommend but I will stress that NO filter or block is 100% secure. Some are definitely better than others.
First of all, I'm not talking about handing it over on a permanent basis, with unfettered and private access. I mean more like handing it over for sessions, which gives relatively little opportunity to mess around. I would imagine that there are in fact kiosk apps (not necessarily free) that are basically foolproof without actually knowing admin login info or having access to a computer to hook up the phone/tablet to - and in those scenarios it'd be pretty obvious that it had been tampered with anyway. CMIIW.
The closest out of the box is app pinning
Definitely an option, but not ideal because I'd have to go and set it up each time. When I looked into this a while back, Fire OS didn't have that (and it's my tablets that are Fire OS, which would work better for this than a phone), but it seems like they might have added it.