We went to Beautiful days pre DC and with them at nearly 1 and nearly 4, then again a year later. We haven't been back as the eldest hates crowds and noise, so no festivals for us for a while.
It's very family friendly but make sure you have change for trampolines, helter skelter, bouncy castle etc. All old fashioned fairground stuff. Kids field is great with face painting, circus skills (free), magic shows etc. My eldest most enjoyed painting someones camper van. Not a random one, provided for the purpose! Mobbed by kids with paint pots. Random wandering street theatre stuff all good fun. There is a toddler/baby area on a hill overlooking the main stage. There was a sand pit when we went. Great way to watch the bands while keeping little ones happy.
Make sure you are well prepared for mud!!! Do you have an off road buggy? The mud nearly killed our 3 wheeled tank. DH had to repair it in the pouring rain and mud. Luckily we had a carrier so gave up on the pushchair and carried the little one. Totally hopeless with small buggies. The mud got really sticky when the sun came out. Remember to bring something to sit on that you can ditch if it gets covered in mud. Alcohol gel, wet wipes and toilet paper are essential. And water bottles for when the sun comes out. Plenty of taps on site for drinking water. Drinks of choice (not in glass bottles) for the grown ups if you like that sort of thing.
There's a proper seated theatre tent where we saw a fantastic (but weird) pinocchio puppet show. Get there in good time for seats.
Have fun. I really want to go now thinking about it.
I can not emphasise enough that you need to be prepared for mud.