I definitely have mattresses in that range; purple is actually the first bed that comes to mind (the original purple, not the hybrids). Each of them in a twin is $599 each and has a 10 year warranty. Great comfortable mattress and it's far better than just buying a memory foam bed in a box. Since it has the purple material in it it's great for any sleeping position (doesn't matter if they're side, back, or stomach sleeper) it's gonna keep them supported and also help alleviate pressure points that could develop if they sleep on a mattress that's too hard. This one would be best for your 2 year olds bed furniture cause it's about 9 inches in height(not a super tall mattress so they can climb in and out of it).

Another mattress that would be more of a traditional innerspring mattress is one that we sell by Serta called the Trelleburg. It's normally $899.99, but we're running a promo right now and they're $539.99 for a twin. I can't tell you how many of these beds I've sold and people love the absolute crap out of them. The one you would want for your kids would be the Firm Pillow Top; it's a medium feel mattress and offers great support and comfort. Has a 10 year warranty and has over 1000 coils in it. Amazing bed for a great price. This one is gonna be a bit taller so it might not be ideal for your 2 year old and would be almost 11 inches in height.

Let me know what you think! If you ever decide you want to do business I can give you a great deal on shipping and mattress protectors! Hope this information helps you on your mattress journey :)