Welcome to the gang, I do a lot of minis rugby refereeing as I coach (U11 currently) so am grateful to you, and I sympathise, because you can do everything right in terms of the spirit, helping the kids, and following the law adjustments (which most other coaches and parents don't know!) and still take flack for it, you need a thick skin, and best I can suggest is not getting too involved in confrontational conversations as it never helps. Explain what you saw, and why you did what you did, or if you didn't see it tell them. If someone keeps going, or you have raised voices then remind them about the example they are supposed to be setting to the kids, they won't like it, but it trumps all else, and more often than not ends an unsatisfactory situation. If you are really uncomfortable then stop the game / ask them to leave, of if post match talk to your clubs safeguarding officer and let them deal with it.
Firstly the priorities I take into every game are safety first, fun second. So any foul play (tackles above armpit, handing off etc), blow straight away is the guidance I was given. Same for anything you're not happy with on safety grounds (lifting tackles, players jumping into tackles etc), better safe than sorry. Whatever you do when you penalise make sure you explain, it's almost never deliberate, and is just a combination of skill, tiredness, frustration that bubbles out. So be sympathetic and explain everything.
Everything else is just kids trying their best, so the best ammunition you have is the very first law in the NROP which in essence says that if it's not material, or doesn't disadvantage the opposition then play on. So knock ons or bobbles at younger age groups, or messy rucks where they over run, or a bit in at the side, or offsides where they've got a bit lost on the pitch, just play on if you can, as long as it doesn't disadvantage the opposition it's all good. The aim is to get the ball out of the pile of bodies and into hands again (hence the pass away at all tackles, rucks, mauls etc). But talk, talk, talk. Tell them what you're seeing, tell them what you want them to do, and explain the decisions with coaching tips about what they could have done differently afterwards. End of the day no one is there to listen to your whistle.
Quite often you will also get mismatched teams, I had one the other week where my team (most with 5 yrs experience) was pitched up against a team of almost entirely newcomers, so I gave the newcomers team a lot more lee way (knock ons, offside, forward passes, playing the ball on the ground) compared to the experienced team, but we also talked amongst coaches before hand, and explained to the parents the situation. If those on the touch line understand what you're aiming to achieve they'll understand in advance and makes your life easier.
However, at the end of the day there will always be a difference of opinion, and you'll always be wrong (and right!), so ask yourself at the end of the game if if everyone stayed safe, and had fun, an you'll have done your job. Yesterday I pinged my team for offside at the kick (3 players in front of the kicker and kicker forgot to chase his own kick, and kids natural instinct to chase the ball in their first 3 months of kicking), had to explain the laws to two of my own coaches, and still they weren't particularly happy, but I felt 3 players offside putting the opposition player trying to catch under pressure was material. If I'd let it go the opposition coach would have grumbled. Luckily I have a fairly thick skin and move on, my wife on the touchline, more offended on my behalf!
Mostly however, thank you for doing it, and please keep it up!