Hello! We took our small for the first time age 3 (he’s 5 this year).
The biggest thing is that while you are there you are primarily a parent. If there’s two of you you can switch off childcare (my husband battles, I don’t, so I’d have the morning/lunchtime slot), but having a child there means what you can do and where you can go is very much dictated by the kid and their tolerances.
I would say a home base of your own in the IC field or very close by in the OC fields is essential. There is a PD provided space - the Academy (aimed at school-age+ but very tolerant of smaller folk), its adjacent Quad (open space, roped off) and a tent/covered space known as the Orphanage which can be used by parents, all clumped together near the Civil Service Hub. However, it’s not open at all times, and can be very busy, so having your own camp to retreat, that your child is comfortable in, is important.
It’s camping, in a field, with a lot of people about. There are environmental hazards (ruts, ditches, stinging nettles, puddles, campfires, attractive shiny cooking knives and utensils). If your little one is very mobile, curious and/or inclined to wander you will need to be on your toes (we were fortunate that ours was quite shy at first and liked to stay near us). A label or tag with an IC contact method - nation, group, IC symbol - and OC phone number attached to them/their clothes is a really good idea.
I would strongly recommend you all be one nation. You can always change later once you’ve more experience of what works for you as a family. As Knave says, having one of you with background in another nation and one or two quirks deriving from that is a nice compromise.
For toddlers you can get away with very basic kit, with one or two nation-specific accessories. Let me know if you’re wanting for base layers as I have a fair number ours has outgrown, and please do ask if you have any more questions!