Boston Terrier would be super for your son

They are small enough to be good with the yorkies, have a great personality and are not "fru fru" dogs.. we have had Boston's all my life and they are great pets!
as for a great dane, I wouldnt want to feed or clean up after one... lol
I have a yellow lab, she is 95 lbs. and is fantastic with my babies. She is truly the best behaved dog we have(of 4) or have ever had.
How nice of your children to ask you about a breed for them!
good luck
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