It's probably harmless. My daughter is 6 and she is always all over the little ones. Not too long ago I caught her in the pasture ON my yearling filly. I can remember when I was a kid putting the pony saddle on the yearlings and tying my cabbage patch dolls to them....Still have one of those yearlings, she's 23 now :)
I'd just be sure they didn't do anything that wasn't safe for them or Rodeo. Though I'd think with the BO being their grandmother they should know their boundaries.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.