Hi Samsmum,
I would completely agree with Jeff that children tend to do much better with firmer mattresses (medium firm or better) that in many cases would be firmer than many adults would be comfortable with and I would tend to avoid softer mattresses for children.
Post #2 here and the posts it links to have more information about mattresses and children including a link to some general guidelines for children in post #2 here.
I would also agree with Jeff that I would avoid the lowest quality promotional or “throwaway” mattresses for children. If you buy a reasonable quality mattress in a firmer range (which are generally available in lower budget ranges) then if they need some additional pressure relief as their body starts to develop into the more “curvy” shape of an adult your mattress would still be a suitable “base” for a good quality topper and you won’t have to buy a completely new mattress when they are older.
In many cases a mattress that would be “comfortable” for an adult that prefers thicker or softer comfort layers wouldn’t be the best choice for a child.
Phoenix