Perhaps after what the mother did, he wasn't too fond of yukkuris in general? Hence, they were yukkuri. Of course, then why did he give them sweets? Maybe he hoped that they wouldn't be the same shitheads and gave them some sweets to help them survive? Why wouldn't he throw them out with the mother, in any case? If you need one though, a small amount of thinking will lead to 'reasons' why.
The mother attacked the owner in typical trash fashion, so the possibility of the children not falling too far from the tree was very much there and Mister didn't want to take his chances with just removing the mother. (Good kids, bad parents and vice-versa notwithstanding.)
Honestly, it was probably done as an author call to get the kids against the parent in a somewhat different way than what's normally done. (read: violence)
Of course, you don't need any reason to do something to a yukkuri. There's not much point asking for reasons or justifications why; most of the time there is none and making some would be a stretch, and you'll just reject them anyway. And then with things like this, a little amount of thinking will reveal some answers. I actually answered it too, and it was the same answer. "Maybe he hoped that they wouldn't be the same shitheads" was getting at that point.
Tea some people may have reasons to do stuff to the yukkuris, in fact the majority have them. Some being complex some being simple.
Hell your "they were yukkuri" is ironically a justification, a simple, maybe not good for some, but still a justification.
So seriously dont' take it seriously, it's not like they asked why the anon "was a douchebag or anything?" or "why couldn't this have a happy ending?" or "why can't we have more ff pics than abuse pics?"
Sometimes there is no justification, or at least one that isn't beyond "well they are yukkuris and we do stuff to yukkuris, nothing more, nothing less", but it isn't that weird to ask in some special circustances and answer properly, instead of recurring to "because he did so" kind of answer.
not the proper, but there are many justifications for this scenario if you want to make them:
"they may had turned out like the mother and the anon didn't want to face the problems of having little shits as pets"
"they may hate the anon if the anon ditched the mother but not them"
"the anon didn't care that much for them and at the first sight of annoyanced he kicked them out without a second glance"
"the anon felt like trolling the little ones out of being relate to the shithead mother, and thought it would be fun."
"he owns them and do whatever he wants with them and if they get bothersome he ditches them like unwanted objects."
You can come out with many reasons and justifications and they won't necessarily be demonizing abuse or anything, it's fun to try to get into what the anon is thinking, to try to come out with a possible reasoning behind his actions, from liking to torment yukkuris to hating shitheads and proactively chastize them, etc.
Hell i bet you have your own reason and justification for the anon to do what he did, right?
I have, simply Anon seen that, Reimu was shit, then he must have thought: If the mother is so, the children are sure to be equal.
or
If I leave the mother, the children will not give me peace..
or
I have no patience to care for them alone..
So many that comes into my head, but as I know your opinion of me, you think I would say something like:
Haaaaaaahahahahahahahahahahahaaa! Anon, saw that yukkuris are worth nothing, and left together with the children, praying that an abuser will find them!!
Looks like the kos are trying to go back the mister's house. The mother will say something against it, and then the kos will probably yell "Stupid mother who can't take it easy with mister human should die easy!"