You won't be able to do everything, bear that in mind. My best advice would be to wander around for the first couple of days, and enjoy whatever takes your fancy. There is so much going on at different intervals that you will just stumble across something which will make your weekend. Try the different foods, explore the healing fields, find the random brass band, the man cycling with a piano, people on stilts, the fun of the Circus area, the Latin dancing in Glasto Latino.
My one major piece of advice would be to check out the sunset from the Glastonbury sign, you look over the campsite and get such a good feeling! (Although you will see many people inhaling balloons, just have a good excuse for the kids!)