If you havent been to glastonbury before nothing can prepare you for the size and scale of the event. It really becomes a village and takes a long time to walk from one area to another.
While camping experience does help, festival camping is something else...Have a couple of practice runs putting up and taking down your tent before you go. Depending on the size of your tent, it might get a little cramped with 3 people plus your gear so it really helps to be organised. I have never stayed in family camping so cant comment on what it is like but as a festival overall it is very family friendly so im sure it will be good. Im sure there are lots of dedicated Glastonbury forums where you would get lots of info.
If you are trying to plan what bands/artists to see be realistic, you cant easily jump between stages / tents so you will really need to prioritise who your must see acts are and accept that you might need to change your plans depending on how tired you are/ how sick of the weather you are etc.
Try to fit everything you are taking with you into rucksacks. It is a real trek if you are carrying your gear any other way. Take as little with you as you can - a few changes of clothes, something to sleep in, minimal toiletries (as these are generally really heavy) and bring a decent comfy bag to carry essentials during the day.
Have something warm to wear as it can get quite chilly in the evening.
For footwear I prefer my walking boots to wellies as they provide more support but you see all sorts of footwear choices from barefoot, flipflops, trainers, boots...
Put suncream on as soon as you head out in the morning and top it up often. You see so much really bad sunburn at festivals. If you forget it, lose it etc you can get it at the welfare tents.
You will have a great time. My only question is whether you might be able to change anything else that week so that you can arrive and stay a little earlier? The first few days are quite magical and helps to ease you into the festival madness.