Okay, if she's the only one under 5yo, her regulations are slightly different to the rest of the 5-16yos, so she may need to be looked after slightly differently.
Maximum number of hours in theatre is 5 (as opposed to 8 for 5-9yos), she has to be out of the theatre by 10pm (as opposed to 11pm) maximum amount of continuous rehearsal/performance is 30 minutes ie time on stage (2.5 hours), maximum amount of performance without a break is 2 hours (3 hours)
Mine have all done panto from aged 5yo and that used to be dress rehearsal Wednesday, Performances Thursday, Friday, 2x Saturday. But the adrenalin carries them through and they don't seem to have any issues. It tends to be the Sunday afternoon afterwards they're awful as the let down comes.
Check what they're allowed for snacks-we only allow water, and no chocolate. That's mostly because of costumes, although we tell them chocolate effects their voices (it does). If you've ever had 10 minutes to get chocolate milkshake out of a Sound-of-Music white sailor costume you will appreciate this. 
Raisins are great, sandwiches with non-messy filling are great for more substantial snack.
Also check what electronics they're allowed. Our council recommends you don't let them have electronics backstage due to photos and bullying on messages. We do allow them, but they're regulated (not in the dressing room, just in the waiting room).
At that age, sticker books are great (no mess), or beads or a small game to play- no felt tips for costume reasons. They may like a scrap book to get everyone to sign their names etc, or a programme to do that in also works well. If she has a soft toy to take along as a mascot, that's great too, but please make sure she's aware it must stay in the dressing room. As a chaperone you can't run back to fetch it from the side of the stage easily necessarily, and tears because you have to leave it in the dressing room is something I've had to deal with before. (and then tears because we let them and they left it at the side of the stage)
Between the performances on Saturday get her off site for a break, even if it's briefly walk round the block. They do need it-even I need it as a chaperone.
If you're doing the make-up then take the spare with you. I've had a few times where I've had to make a decision whether to remove the extra lipstick which is coating about an inch round the lips that they've decided to apply themselves after their mum has done it for them. It's a tough one because again they can be upset if you want to take it off, otoh you don't want the audience wondering why there apparently is a clown on stage with the woodland creatures.
If you do need to leave it, then make sure they ask if they think they need more on rather than doing it themselves.
Have fun and she'll love it!