You get out of a dog what you put into it in my experience. You can't just get a puppy and bring it home, and expect it to be good with children and quiet just because it's say, a cockapoo or something.
Training a puppy is HARD work. It involves getting up several times a night so it can go to the toilet, taking it into the garden every 30 minutes so it goes to the toilet, and cleaning up accidents because even that doesn't't guarantee it won't pee in the house. It involves training classes (expensive) and continuous training at home to get it to follow basic commands, to walk nicely on the lead etc.
Puppies bite, regardless of the breed because that's how they explore the world. They mouth, chew and nip and it HURTS. You need to spend months training them not to bite and even then they'll still do it. They'll chew your hands, your feet, socks, clothes and furniture.
And I don't think 30 minutes walking a day is enough for any dog. Of course puppies need less exercise but as they reach one year, they require a lot more than a 30 minute amble. Most need a decent off-lead run about several times a week, or at least several decent walks. We have a Beagle and he'll need two hour-long walks everyday when he reaches maturity. At the moment he has 2 x 15 minutes due to his age and you can tell he doesn't want to stop!
Getting a dog to be compliant, well-trained and good with children takes a lot of work. But you don't just need to train the dog, you need to teach your children how to behave, when to leave the dog alone and not to get up in it's face or try and fuss it while it's eating or sleeping. You need to encourage calm play and not get the dog to chase the child because it's guaranteed to end in tears.