(ANSWER)
When puppies are young, they will "play growl," but that should've stopped by now at 4 months old. If she is still doing it, even though it may not sound aggressive to you, it needs to be addressed.

I'm sure I don't have to tell you this, but your puppy is very smart, outgoing, and fearless - everything you want in a hunting dog, but it also means she is going to test you. In particular, she's going to test her rank in the pack (your family) and will take full advantage if she thinks any of the other "dogs" can be bossed around. She just tested your son (the 15-yr-old) over her dog bed and likely believes she won, so she'll continue the behavior and will likely start doing it in other situations as well.

The problem you have is that your dog does not know her place in the pack, which is below every human being, regardless of age or size. Because she is older now, it's going to be a bit harder to get her to comply, especially since she thinks she's "won" at least one battle. You are going to need to make a concerted effort to teach her place while she's still small enough for you to have the option of totally physically dominating her if necessary. I cannot stress enough how important it is that you cross this bridge now, and that you win this battle. You do want to live with a Small Munsterlander who believes she is the boss.

The first thing to do is make her lose a few battles with your 15-yr-old son. Use my food "trick." He needs to make a plate of tasty (to a dog) food, sit on the floor, and eat it in front of her. Because she thinks she outranks him, she'll try to take some, and she may even get nasty about it. He needs to be very firm, but not mean (and not yell), and continually push her away saying, "No, mine" until she finally gives up. He should not otherwise acknowledge her presence, pet her, look at her, or anything else while he's eating. He should not praise her when he's finished, nor give her anything from his plate, nor any treat. When he's done, he just gets up and puts his plate away. No treats, no praise, no acknowledgment. He should do this every day until she no longer tries to get some of his food. How quickly and easily she gives in to this will tell you how serious your problem is.

Additionally, your 15-yr-old son should make her get off her bed, then pick up the bed and move it; and/or move the bed while she's lying on it, at least once every day. If she growls, snaps, or tries to bite, he should very quickly and firmly, without a word, smack her in the side of the head and take the bed away from her by pulling it from under her. He should then put it where she can't see it and wait about an hour before he returns the bed to it's regular spot. He should not say a word to the dog when he takes the bed away, nor when he returns it, but she should see him do both things. This will show her that your son is in control of her bed.

Simultaneously with the above, and under your close supervision, you should have your 6-yr-old pick up your dog's food bowl and give it back to her a time or two while she's eating. When he is able to do that completely without incident, up the ante by giving her canned food or a tidbit of people food in her bowl. Be prepared for her to growl, snap, and maybe even try to bite him. If she does any of these things, very quickly and firmly, without a word, smack her in the side of the head and permanently take the food away from her. Let her go hungry. She needs to know there are serious consequences for this behavior.

Once the above is going well, your 6-yr-old needs to eat in front of your dog. You should sit down next to your young son while he does this, so you can be the one to push the dog away and tell her, "No." Timing and control are important for success, and often younger children aren't able to do this by themselves. It will have the same effect if you help your son with this. Also, you should do it with your 3-yr-old niece. Else, you'll find your dog will try to take food out of her hands and may growl and be snappy about it. Generally, once a pup has had this "training" with people of varying ages and sizes, they figure out the rule applies to all people.

I would not have your 6-yr-old do this, but once you are able to do the things above without incident, all the "big" people in your house should occasionally give her something like a dog biscuit or bone and then immediately take it away from her. You all should be able to take something out of her mouth and then give it back to her, without her raising any objections. If she growls and/or snaps, give her a firm "no" and a head slap and then do whatever is necessary to take the biscuit or bone from her. Hold her by the scruff, if necessary. And do not give it back to her.

Unless there's no other option, do not allow her to ride in the front seat of the car/truck. Her place is in the back. This is actually more important than you might think in showing your dog her proper rank in your family pack.

I know some of these recommendations might sound like overkill, or even kind of "mean," but it's really important for you to be firm and consistent with her. Also, once she reaches 5 months of age, I do not recommend you physically reprimand her (i.e.the head slap), even for snapping. After 5 months of age, I instead recommend the dog be removed from the presence of people and her activity curtailed for at least 30 minutes (crated, kenneled, or chained). Kind of like a time-out. That's why it's so important to nip this in the butt now, while she's still young and small enough for you to physically control.

If you do these things, I can just about promise your dog will learn her place and you'll never again have such problems.

(FOLLOW-UP FROM THE OWNERS)
Michelle, we did all of the dominance recommendations you made and she has not displayed any more snapping with the kids. Thanks for your help!