I'm going to tellyou what most the experienced TBF folks will echo. You really want to get there the very earliest you possibly can. Not solely to get a better spot, but music starts wednesday evening. If you show up Friday you'll miss Friday mornings shows, all day of Thursday and the Firstgrass shows Wednesday night.
Mary E opens the Saturday before, and though you'll have no big issues finding a place to park your VW on Friday, it won't be a "sweet" spot, and likely you may be sandwiched between others, but Mary E is pretty spread out, so I don't think it ever really gets tent to tent.
One of the best reasons to get there early, besides all the fun activities that group camps in TP organize, is the seemingly endless amounts of hiking, biking, climbing, fishing etc... that can be explored right from your campsite, or from town. Getting there early allows time for exploring the mountains, acclimating to elevation, and taking advantage of the 9 possible nights of camping your Illium or TP pass allows.
No matter what you and your family will have a blast, just no you'll have less pressure, and have more time to enjoy everything including music if you come early.
Listening to:
Violent Femmes