Choosing a mattress for kids is actually pretty important. A child's spine is malleable and soft, and still developing and growing, which means that buying them the cheapest bed may not be the best bed for their back. So, if price shopping is not the best way to choose a mattress for kids, what is?
One of the first things I encourage parents to do is to do their research about the types of materials used to make mattresses in general. Many of today's mattresses are made with materials that off-gas toxic chemicals, and their bodies have much less mass as our adult bodies. Less mass means the chemicals they breathe in at higher levels in their body, making them more susceptible to adverse reactions to the chemicals. Petroleum foams, plastic fabrics, fire barrier chemicals; all things to consider when shopping.
Another thing to consider is support. Since a child's back is so soft and still in development, it means that it is imperative to find a supportive surface. Don't mistake firmness for support! Support means something that allows the natural curvature of the spine to rest in place without being pushed out of alignment by a sleep surface that is either too hard or too soft.
Longevity is important as well. I usually recommend something firmer, like our Sparrow mattress, as a small child does not have much weight and mass. Then, as the child grows and gets heavier, they are going to benefit from a softer surface. Adding a topper can make the existing mattress softer without having to purchase a whole new bed.
Lastly, you want to consider protecting the mattress. Kids are notorious for accidents and spills, so you may want to consider a puddle pad, washable wool pad, or breathable, washable protector.
Kids deserve to have a nice bed too, but that doesn't mean you need to spend an arm and a leg.