Instead of mixing it *into* the applesauce, try making a thin layer of applesauce with a dent in the middle, put the powder in there, and then cover with another thin layer of applesauce--so it's all one bite. I get my kids to take nasty-tasting powders that way.
You can also repackage into smaller capsules, I've read that dipping them in a bit of honey or maple syrup or the like can help with teaching young kids to learn to swallow pills. DS learned with gel-type capsules when he was, um, 3.5 yo, and that was regular size capsules (started with gel-type, not big, but quickly moved to adult-size caps).
You may want to be cautious adding in calcium, some kids are fine with it, some need it, but it can be tricky to balance in some kids, I think mostly because they tend to need the magnesium so much more.
As a last resort, you can put powder into a strong-tasting liquid, I used black cherry juice for a long time. It doesn't really mix in, it's more a suspension, so it's grainy and doesn't taste good, but with practice, the kids would take quite a few supps in about 2 tablespoons of black cherry juice--the smaller quantity of juice worked better than a larger amount for us, I gave it to them like medicine. I had to slowly increase the amount of the supps I was giving them---really, really slowly. I went too fast several times and had to restart.