I have a leo and a Bernese
I agree with everyone that research is very important, talk to as many different breeders as possible, get to know them and their pups both leonbergers and newfs. before you make your choice
ask as many questions as you can think of.
also it's important to ask yourself a lot of questions write them down good and bad
do you have time for training, pooper scooping, grooming, exercising etc etc that kind of thing. talk to vets also, about vaccines cost, nurturing and spaying cost, reg check ups.
everyone has to be on the same page as you kids, hubby
find out what you need "tools" to train, house train.. like create,toys toys for teething, poop bags food etc etc etc..
and are you ready to take the dog out in the dead of winter /rain/sleat when he/she needs to play or walk??
these might be hard questions but they will be worth your while in the long run