Purchase minnows at a Bait and Tackle store. Be sure to get water from them also – or the minnows will die quickly.
Put the water and minnows in your water/sand table and let the children try to catch the minnows. Explain to them to be gentle and not to have the fish out of the water for too long. At the end of the day – as long as you have been careful – you will need to take your fish to a nearby lake or pond. I’ve done this for 3 years and the children have loved this!