Hi
We got a dog for our daughter, then 14, 5 years ago. We wanted something small that didn't shed a lot and decided on a Jackapoo !
We found a lady locally who bred from her Jack Russell bitch to have a puppy for her niece - the other 3 she sold. We went to the house and my daughter picked one.
It has been a great dog, mainly due to the work my wife has put in, with puppy classes and obedience. But we have have just been lucky!
I think we paid £250 for him, on Gumtree iirc.
In my experience don't underestimate the ongoing costs, time and effort you have to put in, and the small amount of help your children will be when it comes to walking, cleaning up, etc !
Good luck.
Mark