Cavalier King Charles Spaniels (CKCS) are fantastic family dogs - they are small with a mighty dog complex, but their personality is phenomenal!! As puppies they can be expensive, & many breeders hold on to their puppies until they are a bit older (4-6 months). You can also look into breed rescue. If you are intersted in finding a reputable breeder, feel free to contact me privately. CKCS do have some known health issues (eyes, heart), but responsible breeders are aware of these & do testing to work towards eliminating them from their lines.
Schnauzers can be a bit "sharp" in personality. Same with Bostons, but this is just my overall opinion based on hearsay, I don't have any personal experience.
Beagles are great dogs, but they tend to be high energy. Good personalities with children, & a nice size. Many also tend to be vocal. I would not recommend this breed if you are not able to give it plenty of exercise - they demand quite a bit more than their size indicates.
If you go to a pound/shelter, some things to be aware of:
Many shelters do not screen the dog to fit the family, they just make sure the family does not 'disqualify' to have a dog, & then it is typically their choice which dog. Well, you can't know what the "right" dog is for your family in the 1 hour you are there looking @ them, & most don't come in with detailed histories.
So please, be cautious about getting a dog through a shelter. If they have a puppy, chances are there won't be as many issues with the past experiences, but it can still be a crapshoot what breed you are getting. (many many many shelter dogs are mis-categorized into generic breeds - shephard mix, lab mix, etc.) And, not every breed is right for every family. Since you have young children, you need to be more specific about the dog you bring into your family, so that it is the right fit, the first time.
Breed rescue offers opportunities to give a dog in need a home, & many (if not all) will do home-foster. A lot of history is usually known about the dog, & even if not, the dog has lived in a home environment & been exposed to different situations. Breed rescues will also work with families to find the right dog for THAT family.
Good luck! T.