Do a trial run of putting up tent before you go, because that is usually the most stressful part for us!
Depending on how much space you have in car (assuming you are driving)
If you have plastic storage containers, use them for your clothes. One for you and one for DS. If your tent gets wet inside, your clothes will still be dry.
Loads of layers, I really rate wearing leggings and a long sleeve top under or as your pj's, it gets cold at night regardless of how sunny the day is. A hoody is good for sleeping too.
Eye masks, it will be very bright very early
Solar fairy lights can often be fed along the inside, nice amount of light to just take the edge off the darkness going to bed.
An airbed each, side by side, is more comfortable than a double. The smallest person ends up being bounced off by bigger person moving around on a double. A sleeping bag each and maybe fleece blankets. We bring pillows, but if you don't have space, bring a pillow case and stuff it with spare clothes
we have a camping toilet to avoid trips to loos during the night.
A small basin is handy for having a quick wash in your tent, and for bringing dishes to wash area
Any specific festival questions, ask away, we go to a couple each summer.