We've been taking our two to a festival every year since they were those exact ages and they're now in their teens!
Greatest thing in the world is a trolley! Cushions and blankets, something to soften the bottom. They would both sleep in there come the evening and have day time naps in it too. All rules on food / showers etc go out of the window for the weekend and just go with the flow. Most festivals, even tiny ones, have great kiddy areas now with activities and events especially for them.
OK, we weren't down the front leaping around when they were little, normally more around the edges so people wouldn't fall over the trolley (glo sticks or fairy lights on it to make it visible when dark is a top tip - decorating your trolley is quite the competitive sport at festivals too!). Camping chairs are a must - a few times I ended up watching a band on a big screen while trapped under a sleeping child!
Oh, and if they have a family camping field, try and camp there as you're much less likely to have the late night / early morning partiers!
And re wristband with phone number for toddler.. I sharpied my phone number onto one arm, and a "if I am lost, take me to first aid tent please" onto other arm as mobile signal at festivals isn't great!
Oh, and a really good cool box - an icy tek, or coleman xtreme - is a god send!
If you go with an open mind you will have a blast! Like I say, we just relax all rules for the weekend