The treasure hunt in the yard is a great idea - I've done this with mine when she was too timid to venture out.
With my pup, I try to start the play and end the play BEFORE she looses interest. This doesn't always work, since she often will get distracted very quickly. When this happens, I say "take a break" and walk away for a few minutes. Then when she seems interested again, I say "let's play" and start the games again.
Treats also work great at getting a reluctant dog to follow someone "new" around the yard, someone other than "mom".
Before you leave for vacation, you could have your kids stand next to you while you play with your pup, and let them handle/throw/tug the toy a bit, so she learns that other people are fun to play with, too. I have to do this with my own kids, too, since my dog only likes to play with me.
I did have a second dog for a while and they played together quite a bit. Now that the older dog is gone, she doesn't really enjoy being out in the yard without a human with her. She stands on the step staring at the door, waiting for me to let her back in.