A lot depends on the age and maturity and responsibility level of the children, and of course, your living situation. Are you in an apartment or house, is it large or small, do you have acreage?

I wouldn't recommend a small dog with young children, as the smaller breeds can tend to be nippy and snippy. Young children can also unintentionally injure a small dog, as some are very fragile.

And of course if you are in an apartment setting, I don't think a St Bernard or Newfoundland work out well in that setting either.

Your best bet is to do some research on line, and also visit a shelter or rescue. If this is your first dog (and it sounds like it is), then a medium size mixed breed is probably the best choice (depending on the mix of course).