The correct firmness for a child is something that we discuss a lot among ourselves at Spindle. We feel either our Medium or Firm is a great place to start. It will feel firm enough for a child while retaining a tad of cushioning.
There is no scientific or medical data regarding proper support and comfort in a child's mattress. Most of what we have seen is anecdotal or pseudoscience. The only rule that we are aware of is having an extra firm mattress only matters with a crib mattress when you have concerns of SIDS and suffocation from too soft of a mattress and inability to move for very young children/babies.
Here are a few other things to keep in mind when buying a bed for [most] children.
1] Most kids are going to love the bed no matter what firmness it is. They're just excited to have their own bed.
2] You'll likely be spending plenty of nights cuddling with your little one. We recommend you choose a mattress that you'll also be comfortable in, too.
3] We strongly recommend a waterproof mattress pad or cover for your child's mattress. While the top of the mattress zips off, it is not washable nor can it be dry-cleaned. In the event of an accident it's better to wash the the mattress pad than having to buy a new cover for the bed.
4] Check out our website to make sure you have a proper support system for the mattress. Since latex is very heavy and flexible, it requires a sturdier base than conventional mattresses. An improperly supported "all" latex mattress will try to compensate for the missing support and the result is a softer, less supportive feel and possibly provide a saggy feel.