What riding do you do? i.e. mx, trail or what? What sort of bikes do they have?
Gloves & goggles are cheap(ish), so easier to get all 3 at once. If any of the kids are on quads, boots won't be as important at that stage (with trail riding, not as easy to hit stumps or get the foot stuck in the rear wheel). It really does depend what sort of riding you do though.
Keep an eye on the for sale forum as I've seen kids' stuff come up sometimes (be quick, it sells quick enough or maybe place a post there yourself).