So we're U13s this season, but have been using the old Continuum variations up until now so have had contested scrums for the last three seasons. I had some great words of advice from the guys on RR before my first season of them and, whilst I don't consider myself sagely in any way, shape or form, will gladly pass them on to you, too.
I found there was an awful variation in the way scrums were reffed (still do...) and a lot of refs don't carry out a pre match brief, which I think is an essential part of ensuring safe scrummaging.
My top tips would be:
Pre match brief with the forwards, explaining you will control the formation of the scrum with CBS, looking for nice long binds from the props, and everyone to be nice and straight, tower of power, etc. Emphasise you will be reffing with safety at the top of your priority list, if anyone has an issue let them know you want them to shout loudly and you will blow the whistle. Everything stops when you blow the whistle. No pushing till the ball comes in, no pushing more than 1.5m (which I presume is still the case this year, although I can't find reference to it in Appendix 7).
At the first scrum, "hookers left of the mark", remind them of the points you made before the match, and reiterate no pushing till the ball comes in.
Control the engagement (the amount of times you hear "crouchbindsetyesnine!!!", even now, after three seasons of contested scrummaging horrifies me), make sure you're happy with body positions, etc before moving on to the next stage. After set, it's worth checking both sets of props for long arm binds - even going around to the other side to check "wait for me, 9, I'm just checking binds". This will will go some way to ensuring safe, stable scrums.
Once the scrum is up and running, keep the whistle close to your mouth to blow up as soon as you see a collapse, wheeling, etc.
As the game goes on, and they get more scrums under their belts, let them know how well they're doing, have a quick word on specific issues if you need to ("tighty, can you just pop your inside shoulder for me", "longer bind, please, loosehead", "get your bum down a bit hooker", or variations on those themes, tend to be the most used phrases for me...). Keep everything light and positive and they respond really well.
Dont be afraid to reset if you're not happy, safety-wise.
...and don't forget the sneaky half backs taking advantage of your diligence towards scrum safety, as they blatantly encroach their respective offside lines whilst you're not paying attention to them!