If there's a calm pool or slow moving stream on your planned day hike with the kids, consider taking a swim or wading in.
You’ll then want to bring bathing suits, swimming trunks and towels. Wear flip-flops or a shoe (but not the shoes or boots your children hike in) when in the water because sharp rocks or a stick stuck in the mud may sit at the bottom.
Also, always make sure the water actually is clean enough to enter; some waterways are polluted, though generally the higher the elevation the safer the pool or stream.
Read more about day hiking with children in my guidebook Hikes with Tykes.