this is a good option. some breeders have social friendly adults needing to be placed, could be a good option.
i wouldn't recommend fostering per say. Unless a rescue has a system wehre dogs are assessed over a few weeks then placed into foster homes. More often than not they NEED the foster homes to do the assessment. not ideal for a home with limited experience and two young kids. Fostering is great but it's not always the best option.
I agree with the comment that adding a puppy is like adding another baby. it's a ton of work. and you also need to work wtih the kids to train them on how to behave with the dog (ex: even having the kids playing and the dog running around the yard will take some work to enure everyoen is safe). And the pup will need exercise daily (less so at first but from 6-18 months think an hour a day - rain or shine, hot or cold).