Constipation and little kids

I know this is very common in kids but my little guy struggles with this. There are three magic foods: spinach, homemade kefir , and beets. Although he likes all three I can’t give him them every single day right? He always goes the day after having those. But broccoli, cabbage , peas, and any other fruit or veggie does not do the trick. I just put him on a magnesium supplement low dose and it’s doing the trick perfectly but I realize I shouldn’t do this long without asking his pediatrician so that’s what I’ll do next. Any other things that help your kids that are natural and not miralax? . Obviously water intake and a variety of fruits and veggies which we already do. Exercise. Anything else? I may just do kefir every day but he only drinks it with jelly mixed in which is sugary of course. Do probiotics help? Haven’t tried that yet.