According to allan allan at his dot com:
For granddaughter, considering :

Rubber chunks (recycled truck tires) very expensive and some issues
with toxicity.

New product called sofplay - it is some sort of astroturf like with
mat under it

Wood chips - mulch

Look up some chart recommendations for many materials w.r.t.
"safe fall distance".

Figure what the maximum fall height is, the charts will
make suggestions as to what they think appropriate for that height,
and work from there.

Strange as it may sound, a 4-6" bed of pea gravel is just about as
good as vastly more expensive solutions like rubber. 8-10' fall
distance, doesn't grow anything, and doesn't attract cats.

Sand, depending on the type and how compacted it gets, is almost
as good, but it tracks, can grow things, and attracts cats.

The natural sand unders ours is always quite loose, so we didn't
have to bother. Didn't track, because it had grass on it ;-)
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.