It looks like rubber mulch.

Hi and thank you for your lovely compliment. We used rubber chippings, they come in a variety of colours but we found the red to be the most natural looking. They’re an amazing alternative to bark. They’re clean and cats don’t use the play area as a toilet. The kids love to dig in it. Perhaps the only downside is having to pick them off the grass once the children have been playing but that really is the only negative.