I think that for the next few park visits, you should guide her play. Maybe arrive with some sand toys and make suggestions like, "Let's see if we can find anyone to share our toys with today." Or things like "I hope there will be lots of kids for us to play together on the slide." Maybe if leading up to arriving at the park, you can mentally prepare her for the idea that other children will be there and the park, its structures, and your toys are all meant for sharing.