Translation:Children are playing with sand on the beach.
Hi, the verb 'bawić się' takes the instrumental. Here the children are using the sand as an instrument in their play; the preposition z isn't used.
'With sand' sounds a bit odd to me. In this context I would expect an English speaker to say 'playing in the sand', which implies playing with it.
Added 'in the sand'.
But it seems that people don't have any problem with the 'with' part, I see only one report from 7 months ago, and one freewrite report which I assume may be yours.
The children are playing and they are „using sand to do it” / „The sand is what they use to play”.
Well sometimes this concept can be expressed by adverb: Powiadomiła go telefonicznie. (She has notified him by a telephone) - I don't know if English can express it by using adverb at all. A Pole can also say: „Powiadomiła go przez telefon” („by telephone” lit. through a telephone). And also powiadomiła go telefonem. (Here telefon means a telephone call rather than a telephone). And „Powiadomiła go telefonem marki Samung” (She notified him by using a Samsung telephone.)