I think you made a nice decision. If they lost a dog recently, they're probably hurting a bit. A kitten that you guys have been caring for is likely to be a very nice animal. Just seeing your posts tells me that you know how to work with animals and would socialize a kitten very well. By giving this kitten to them, you're helping them overcome their loss. I'm sure that it pains the girls, but as Bryan mentioned, you're setting a good example of helping others.
This may be a horrible thing for me to say, but if they aren't experienced snake owners, there's probably a 50/50 chance that the boa constrictor will be back anyway. If they are happy with him, then again you've spread good cheer. There will always be need for someone to rescue boa constrictors. They're beautiful at a young age, but many people aren't going to be able to handle them as they approach adult sizes. Your investment of time is hard to let go, but you've done a generous thing.
A buddy of mine went through something similar around Christmas. He parked at a Burger King and heard a kitten in a dumpster. He talked to the folks who worked there, and they mentioned rescuing 7 kittens from the dumpster earlier in the day. This one had fallen between things to the bottom and hadn't been noticed at first. A kid who worked there came out and dumpster dived until he found the kitten. (My buddy is 42 and has had over 40 operations of various kinds. He can't dumpster dive anymore.) The kitten was soaked to the skin and looked horrible.
My buddy took the thing to a vet who pronounced it miserable but healthy. My buddy took it home and listened to it howl for about a day. Afterwards, the kitten was fine. It was friendly and playful. He'd come home and sit on the couch. Within a few minutes, he'd hear the thing bouncing down the stairs. He'd cross his legs to get a foot in the air. The thing would jump on the underside of his foot and wrap its front legs around the top of his shoe. Then it would push up with its hind legs and ride his foot like a yo-yo. After other kitten stuff, it would fall asleep on the couch with its head on his thigh.
He was on the edge of keeping it. He thought it was a great cat, but his health and job situation has been tenuous. When a nurse at his dialysis center said she certainly wanted it, he gave it to her. Fortunately, he didn't have kids to feel that separation, but letting go of a great little animal was tough. The good news is that he knows it was going somewhere good.
I admire the way you folks can work with animals. I think it's great that you can share your home with those that haven't done so well in encounters with people. I wouldn't have that patience.
Best hopes,
Bill
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It's not how many snakes you have. It's how happy and healthy you can keep them.