Pets are great for kids.

There are TONS of options out there.

As for dogs, certain breeds tend to be better than others. I would suggest a lab, if you have the room, as they tend to have very kind temperments (keep in mind ANY dog can become agressive and must be trained to be gentle). Beagles can also be very kind, but they DO like to howl at things and bark, and need to be excersised quite frequently.

My aunt has a Nova Scotia Duck Toller crossed with we think a border collie, and he is very gentle and kind.

Myself I would stay away from the little dogs both because they can be very yappy and because the smaller breeds tend to not have as good a temperment in situations with kids. Especially poodles, jack russel terriers... but of course every dog is different abd sometimes these breeds do just fine anyways.

If you can afford it doggie school is a GREAT thing for dogs to learn the basics training.

The other option for you is to adopt a dog that is slightly older and has a good reputation with children. however, USE CAUTION when doing this - because you never know when a dog may have been abused or has certain triggers from the family it was with. Most shelters though have a try it out period where you can take the animal to evaluate him. Also try taking your kids to meet the dog, be it from the shelter or from anywhere else.

it is COMPLETELY possible to train a dog to be good with children. I would be hesitant to get a puppy with the girls being so young right now though - it would be a big job to watch a puppy and them and your nephew all at the same time.

Cats are a good option, if you can get a kitten. This way they grow up around the kids and can be socialized to suit the children very nicely. I wouldn't go with an adult cat, because often they will get very cranky with kids antics if they haven't been properly socialized. Kittens however adapt pretty well.

Rats are AWESOME. I reccomend Dumbo rats or Hooded rats. Hooded rats are very smart and can become very affectionate. Here is the thing about rats though - Rats really should not be kept in singles. They are very social creatures and often don't do so well when kept alone. Getting two hooded rats, I reccomend males personally they seem to be least agressive. And really, two rats don't take up much more room than one rat, and really aren't much more care.

There are other options too - some reptiles can be good, although I almost never reccomend a reptile for a young child, both because of the salmonella risk and because they can be particularly delicate.

other options include bunnies, guinea pigs, mice

let me know if you want more help figuring this out