The Allswell Supreme 14" Hybrid Mattress

IKEA is pretty cheap.  The Walmart brand Allswell could also work since it's also sold in their stores.  I've had issues getting Allswell to honor their warranty, but their cheaper thinner mattresses probably won't have the same foam sagging issues.  The actual quality and comfort of the mattresses are pretty good, especially considering how much they undercut the other online brands.  

I’m not sure where you heard that kids need firm mattresses but that’s not accurate. Kids need supportive mattresses which doesn’t necessarily equal firm. Too firm or too soft both will not offer correct support. On average, the lighter the body weight and softer the mattress but try multiple in person if you can. I would avoid the Zinus. They have a bad reputation and they’ve earned it. They’ve been plagued with issues from fiberglass to switching their foam suppliers resulting in inconsistent quality. The Sealy has better coils but the Allswell has better foam. The Sealy will feel more like a traditional mattress and the Allswell will feel more like a true hybrid. If you tend to sit on the edge of the bed at all pick the Sealy for the much better edge support. If your kid has a tendency to jump on the bed, the Allswell can make that a little harder for them to do. Either way, get a good mattress protector so pee and sweat doesn’t ruin it. Kids destroy mattresses like no other.

Allswell, Ashley Sleep (Chime), or Zinus? For a 10" hybrid medium firm they'd run about $200 and would last even maybe 3-5 years. (I just got an Allswell so I can't vouch for the duration yet besides other reviews and build). Keep in mind they may lack some "extra" features such as motion isolation or added edge support. Maybe you could get something purely memory foam with better features though it would sacrifice duration

The allswell brand from from Walmart was inexpensive and very comfortable.

My husband and I have been sleeping on an allswell queen mattress from Walmart for years and really like it! The twin is $192.

I have an Allswell queen sized hybrid mattress that I have now owned for nearly 10 years and it is still as comfortable as the day ai for it properly set up. Note - not the day I bought it, that was a very uncomfortable night. The mattress currently costs $389 - I think it was $199 or 219 when I got it. Yikes. I ordered a metal frame for it to go on that ran me about $100. The first night it was much too hard for me - supportive but had no give for my hips and shoulders to sink into when I lay on my side. I added a 3" cooling memory foam and then a mattress pad that has a little bit of padding on it over that. That ran me maybe $150 total when I originally purchased it. And it made all the difference. The mattress is firm and supports my weight well but the memory foam topper has just enough give to sink into it a little without it becoming this mushy pit that my previous pillow top mattress very quickly turned into As the firm mattress underneath has held up great. I've ranged in weight from 220 to 1t0 lbs as a petite woman and the mattress has been comfortable at all weights. Depending on my current weight and how my back is holding up I have slept on my back, on my side, and even face down into the pillow on this mattress and once again it's always been just the right amount of supportive and comfortable. I've had house guests sleep on it who are considerably heavier than me and they've said it's a great comfortable bed as well. I've actually recommended this mattress/foam topper combo to a couple of friends who purchased it and loved theirs as well. And honestly the fact that it's almost 10 years old at this point and there is absolutely no sagging or mushiness anywhere on the mattress? That is the most amazing part of it. I've had to replace older mattresses every 5 years because they just turn to mush, and every one of those has cost many many times what this one did. Yes I know I sound like an ad for it. I promise you I'm not. I just really really really like my bed.

I have a cheap allswell hybrid and it seemed to he pretty good for my spine. I feel like a mattress with a 6 to 7 firmness would be good all around.