First time pet owner with a child? My first inclination is to warn you like the websites have done.
I don't know why you want a yorkie but you need to dig deep and ask yourself that question. Yorkies are wonderful pups, but the websites are correct when they say they are not known to be great with children...does that mean it never works out? NO absolutely not...there are many wonderful homes with children and yorkies. Just don't discount what is said....
They are terriers and can get annoyed easily by children. This person is telling you this pup is good with children...I would ask how they know that....if based on a couple of exposures to children, be very careful. Issues often don't show up in pups for 2 - 3 weeks.
The other issue is that children, and sometimes their friends, want to pick pups up and love on them....not a good idea. Yorkies are fragile and they can end up with broken limbs...or dead from falls on their heads.......and again, some of them don't like being smothered.
I would advise you to think long and hard...which it seems like you already are ... good luck and if you do decide to adopt let us know and show us some photos!