I currently have both my boys (ages 8 & 6) on Simmons Beautyrest Recharge Classic luxury firm mattresses which we've been very happy with.
I now need a mattress for my 3 yr old girl. Her daybed though needs a mattress that is 10" max. Do you have any recommendations?
Your boy's mattresses, typically, will last about 10 years.
If you plan on moving them into a bigger mattress, once they get a little older, then what you have will time out well.
However, if you would like those mattresses to last until they are grown and gone, add an EverEden 2" Soft or Medium Vytex Dunlop Latex topper.
As they get bigger and heavier, the latex topper will absorb their body weights...allowing the mattress to last a whole lot longer then they normally would.
Those latex toppers, shown here:
Soften the surface of a too firm mattress. Insulate yourself from the memory foam in your mattress to Sleep Cool. Extend the comfort life of your mattress.
On this next link, you'll find all latex and latex hybrid mattresses.
Any would be perfect for your daughter...providing excellent support, comfort and longevity....and all are 10" or less.
My grandson has one of these.