My 11 year old, diagnosed at 8, has been using the Dexcom for at least 2 years now. I cannot tell you how much piece of mind it brings me. I can actually sleep through the night! When she plays sports, swims, has friends over, etc I can just look at the receiver and know what's going on with her blood sugar. I know if I need to pull her from an activity to have her test, give her a snack or bolus her. And best of all I know when I DON'T have to do those things.
It also allows us to have much tighter control through the night. Without the Dexcom I wouldn't put her to bed under 180, with it I can comfortably put her to bed when she's at an even 120 or even lower.
The alarms wakes me but they don't wake her so I can get up and treat the high or low without her even knowing it. The benefits go on and on. I'm a nervous wreck when she takes a "Dexy break".
As far as issues go, we haven't had many. The sensors don't stay on very well so we battle with that the most. My daughter carries her transmitter and pump and cell phone in a flip belt but they don't make kids sizes so that won't work for you for a while (the xs is almost too big for her). When she first starting using a pump she carried it in a spy belt. They work really well.
You can set the low alarm to 100 so it will alert you before they get too low but beware that there is a delay with the dexcom so if they are dropping quickly it may not alert you before they are low. Luckily my daughter feels her lows so that's not a problem during the day and the quick drops usually don't happen at night since she doesn't normally have a lot of insulin on board at bed time. It will take a few months to understand how it works with your kiddo and for you to trust it but once you do it will be hard to go back (at least it is for me).