Accessible (and educational) deck building game for second grade primary school child?
If you're looking for simplicity, probably one of the easiest to learn would be Star Realms. It's the simplest deck builder I've played, at least for the base game, but still provides some depth. 

One other game I've seen lots of kids enjoy is Quarriors. Your "deck" is composed of dice in a bag, but the same concept applies--you accumulate money to buy creature dice, and those creature dice deal damage to other creature dice. A lot of symbols to deal with, especially if you buy the expansions, but still very accessible. 

Getting to math concepts of tens/hundreds and negatives might be difficult since deck builders tend to deal largely with single digit numbers and don't usually have "debt" systems or the like, but the money counting/addition/subtraction element should be present in almost any deck builder.