Hi ODE,
This thread along with post #2 here and post #21 here may be helpful to you in terms of some general guidance and suggestions.
Out of the two mattresses you suggested … I would definitely suggest the Ultimate Dreams because you can choose the firmness of the comfort layer (and you can talk with them to help decide which would be better for a child although I would tend to go with a “medium” firmness choice) and it has latex in the comfort layer which is a much higher quality and more supportive material.
There are some good lower cost foundation options in the foundation thread here. If height is an issue and even a low profile foundation is too high then a good bunkie board (along the lines of this) would also work.
Some of the better local options in Denver are in post #2 here if you want to look locally for more natural materials but if you want to keep it simple then the Ultimate Dreams with your choice of firmness level would be a good choice.
Phoenix