Tbf Kid doesn’t care at all about Momo so it’s understandable why he wouldn’t help.
Law didn’t try to help though because Oda’s turning him into a tsundere
Not really, Oda's simply doing what he always does, reverting characters back to their original state after their character arcs, (which is why WCI was so pointless for Sanji). Law was always a cold dude, not some goody two shoes. He had no issues with the kids being guinea pigs back at PH, as long as he got what he wanted from the situation, he'd continue to let them suffer.
Earlier this arc Hawkins even let his crewmates go because he saw Law (likely via his cards) was willing to kill them to get to Hawkins. People are just being reminded once again who Law is, and they don't like it because they were under the illusion that he was a strawhat or something.
Luffy is the real issue this chapter anyway, went G4 to take down a ship in a pissing contest with two other dudes who can do it by themselves, but not willing to exert some energy to even
attempt to fly up there and take Momo back from being an unnecessary hostage.
Sanji has been a simp, so atleast he has that to fall back on, as cheap as that was (like Nami needs to be protected from that shitty attack, like there isn't any other SH who can do so) atleast there was an attempt, let's not forget Sanji can go invisible though, don't even get me started on Robin.
This whole situation could have been avoided if Oda just had let Kanjuro escape before the Strawhats could react, instead he let them see all that was going on, in time for some to attempt, and others to just sit there and do nothing or hold conversations with Momo. Horribly handled.