I haven't done it before, but exactly the same situation and doing it this year with an 18 month old. I'm planning lots of snacks and a couple of toys for the coach, then just hoping she sleeps for some of it... We're taking a bike trailer to use during the festival, so hoping that putting our stuff into that and having her in a back carrier will let us carry everything across site, though Wicket family camping is just inside the coach gate if we decide to stay there.
We have friends with general admission tickets so they're actually going to drive our car there, then we'll drive home and they'll take our coach space. The main issue at the moment is (despite charging £62 for a baby to travel) seetickets aren't providing a carseat, so not sure how we're going to carry that once we're there. Current plan is to try to manage it in the queue then stick it in a lockup just through the gate.
Definitely keen to hear tips from anyone who's done it before though.