Hi spup345,
Welcome to the Mattress Forum! 
You may have read this already but just in case you haven’t, Post #2 here and the topics it links to have more information about mattresses and children including a link to some general guidelines for children in post #2 here. It also includes a number of links to the better forum posts and topics about mattress and children as well which have more information about many good options for children which may also be worth considering.
There is more about the pros and cons of one sided vs two sided mattresses in post #3 here. While there isn’t any way to quantify how long any mattress will last for any specific person … it would be reasonable to expect that if you flip a two sided mattress on a regular basis (see post #2 here) that it would last 50% or more longer than a similar mattress with a similar support system that used the same comfort layers on only one side of the mattress.
Both of these use good quality materials and components and are in a medium-firm range so they would both be a suitable choice for a child. While pocket coils are more contouring … this doesn’t necessarily make them “better” for a child although as you mentioned it may make some difference when they get older and heavier and develop into their more adult proportions although it’s possible that both mattresses may need a topper when they get older if they prefer a softer mattress. The Berkeley Ergonomics also has the option of adding an additional layer of latex which would also provide some additional contouring for older children and teens.
The Pure Echo here is also a similar mattress made by one of the members of this site. It also uses a pocket coil along with wool and cotton comfort layers and is also available in one and two sided versions and may also be well worth considering.
Phoenix