leave a bag of dry clean clothes in the bvehicle as if it DOES pour down, the whole time, it means you have something stashed away/Amd if it is the type of festival where you cannot get back to your car, it means yu at least ahev some clean dry clothes to travel home in.
Most family friendly festivals have special areas for toddlers - find those and familiarise yourself with what's avaialble. Many have nappy change tent etc or an NCT presence.
Hook up with other parents camping nearby - it is always worth having an extra pair of eyes and ears/someone to play with etc.
Get one of those £10 sunshelter tents and you can park up at the back of the main field, and put blankets etc down as a crawl hole for tired kids to allow you an extra hour or so of music watching time.
Buy glowsticks BEFORE you get there, much cheaper and invaluable for nightlights, and child spotting when it gets darker.
do not stress if they don't eat their normal diet for the weekend - it won't kill them if all they eat all weekend is bread and biscuits
If you have to camp away from your car, think about a foldup wheel barrow to help transport your kit to camping area.
Baby wipes/babywipes/babaywipes!