First, start this way:
http://www.shirleychong.com/keepers/archives/bite.txt
Next, this puppy should have no time unsupervised with your children. Period. Unsupervised puppies learn bad behaviors that you end up having to undo later.
Have supervised play time each day, teaching the PUPPY how to play with the kids, and vice versa.
Arrange now for puppy training class. Your puppy should start NO LATER than 16 weeks, and sooner is better. Look for a positive motivational class. Stay away from the collar yankers.
And last but not least, READ. Read read read read. There is an unbelievable amount of info on the net.
Get to know your friendly search engine.
Search Puppy Training, Bite Inhibition, House Training, etc etc.
Read the stickies in this training section, and keep asking questions.
You are going to be one busy person with 3 kids, a newborn, and a new Lab pup.