The sand discussion is a perennial one. If your plants are doing well, leave them alone. As you try new and different species, sand will become important, and the quality of it as well. The best as has been stated is pool filter sand. The safest is pure white sugary looking sand: no mixed colored grains. The problem with "play sand" is that there are lots of plces "play sand" is mined, and the chemical composition varies greatly. I stress the need to maximize all aspects of good CP cultivation. This often means taking no chances that I don't have to take. Maybe play sand is good, maybe not. This is too chancy for me. I know that all the pool filter sand I have ever seen is silica sand, and I ASSUME this will hold true across the country, more so than play sand. A good test is to put the sand in a clear bottle and add vinegar. Observe the sand over time and if no bubbles form you are looking good.
Beware of substrate terrariums! They look great, but over time nutrients accumulate and they inevitably (in my experience) decline. I lost an entire collection this way and will never grow in a substrate terrarium again. Better to plant in pots and bury them and landscape around them. Remove them once a month, flush the pots by top watering and replace them. Avoid the use of wood or other dead plant material which is host to fungi and always use PURE water. IF you are going to do a substrate terrarium, always use *distilled* water, and be sure to remove dead material asap. Even with these measures, plants will decline over time. My terrarium lasted 10 years, and I was religious in its upkeep. Most won't last that long.
I am big on growing outside of terraria where conditions are dynamic, not static. The plants respond to dynamic conditions and languish in overly humid and light starved conditions. Good luck with you terrarium!