Suggestions for Horseback Riding for kids

My niece's 9th Birthday is on Sunday and she really wants to go horseback riding. My sister is having a hard time finding a place on the island to go because a few places are saying that my niece is too young

Also for a few places she keeps getting an answering machine....She wants to talk to someone because although we are not looking to have a birthday party there, we will have a pretty big group looking to ride.

Can anyone help me out with any info? Thanks!!!