If Darling and Jim Dear aren't in the mood the celebrate on the Fourth of July since Scamp is missing, why did they come to the celebrations in the first place?
Perhaps they thought they might find Scamp there.
It's implied that Aunt Sarah talked them into it.
Tying into this, Aunt Sarah prolly thought it'd cheer them up.
Abandoned within a year?
Angel mentions that she has had five families before becoming a junkyard dog, and that she had to leave one of them when they had a new baby. I know it's not specified exactly how long she spent with each family, but if you think about it, Angel is clearly around the same age as Scamp, which means she can't be any more than a year old. So did Angel leave the family who had the baby when the baby was born (which, if that is the case, the baby's mother must have already been pregnant when she moved in with them), or did she move out when they family found out they were having a new baby?
Since five goes into twelve twice, she could have been with her families for about 2ish months before getting passed to the next set of owners, in which case, she's been recently left to the streets. That said, as far as a "new baby" being one of the reasons, she could have been neglected or abandoned either after a baby is born or once a pregnancy is announced.
Buster's Domesticated Roots?
While a canon breed hasn't been officially stated for Buster, he appears to be a Rottweiler or a Doberman, or perhaps a mix of the two. This guy prides himself on being a street dog and the leader of the Junkyard, supposedly never had a human owner in his life, but look into the natural ears and tail for the two breeds above. Both Rotties and Dobermans have naturally standard-length tails, Dobermans have naturally floppy ears. Buster's ears are somewhat upright, but the tips droop somewhat, his tail is clearly docked. While it could be a possibility he lost it to an injury or something, there's no visible scarring and it looks like a clean cut. Maybe he's a Doberman that was initially bred to be a pet (hence the slightly upright ears and docked tail), but ran away before he could meet the aesthetic standard and has lived on the streets ever since? Maybe Buster does have a past with people, but he doesn't like to talk about it?
Memories
Tramp at one point surprises Buster and the other junkyard dogs when he proves that he still remembers some of his tricks from his street dog days (by effortlessly opening a locked gate the others had troubles with). As mentioned above, however, it's only been about a year, perhaps even less so since he left the streets; why would he have forgotten anything that quickly?
Buster had an extremely low opinion on Tramp after the latter left the streets to become a pet. As such, he probably was under the spiteful assumption that Tramp had lost all of his tricks as a result of his new life. The others may very well have just been following his beliefs.
Size
A case of Writers Cannot Do Math, most likely, but Scamp and his sisters should be much bigger. If they were old enough to be crawling around at Christmastime (as they were at the end of the first film), we can assume they were no younger than three or four weeks old. By the Fourth of July, they'd be between seven and eight months old. They might not be quite full size by that time, but they'd be darn close. note That is, of course, assuming that Tramp doesn't have ancestry from much smaller breeds and managed to pass that particular trait on to the entire litter. Seeing as all four puppies are clear copies of one of their parents rather than mixing their traits, it's unlikely anyone put that much thought into their designs. Angel seems to be the same age, but at least she has the excuse of most likely being a small breed (the Disney Wiki states her to be a Pomeranian mix).
Regarding Scamp, it's possible that he did, if fact, inherent a smaller size from his mother, who herself is of a rather small breed, and that Scamp actually is close to his full size. This would explain why he sounds like a teenager while still looking like a young puppy — he's just smaller than his father.
