Will play sand kill the potted plants? My son mixed a handful of it in the pots.

The addition of sand to potted plants won't damage the plants. However, depending on how rough the plant/roots were been treated when the mixing occurred, they may be a little stressed. Ensure the plants are well-watered and if they look like they're a little under the weather, you can gently feed with Yates Dynamic Lifter Liquid to help. Click for more information: