I’ve seen some people say that Luffy is completely against children joining him, because Shanks declined to invite due to him being a child, though I don’t think that should obligate Luffy even though he looks up to Shanks.
It’s not just that Shanks did it, but that one of the main lessons that
Luffy himself learned in chapter one was that Shanks was right and the sea is too dangerous a place for someone who can’t properly protect themself. It’s not “I won’t let kids onto my ship cause Shanks said so”, it’s “I realised as an eight year old (same age as Momo and Tama) that the sea is no place for someone who can’t protect themselves.”
Ace also said the same thing to Tama when he rejected her, that a pirate had to be strong. I don’t think ”relying on animals to fight for you“ is what he meant by that.
Tama and Momo both are needing to be protected right now because they can’t fight by themselves. Tama’s schitck is that she gets animals to do her fighting for her because she can’t actually fight. Momo has been training, but we’ve yet to see anything that suggests he’s not still just at a kid’s level in strength.
I can see Luffy promising to take them to sea one day or even doing something vaguely Shanks like and inspiring them to do it themselves in the future, but as the two are right now- if the message of chapter one is consistent- they can’t go with the Strawhats.
Regarding Law, for those of you who don't think he will join, how do you think he will find out the meaning of the will of D? The Strawhats are certainly not going to tell him given how much Luffy values the secret of Raftel, and the only way he's getting there himself is on the pirate king's ship. I keep hearing that he would never abandon his crew, but Franky abandoned the Franky Family and Jinbei abandoned the Sun pirates (who he went through a lot more with than Law did with his crew), and I think if it becomes clear that Luffy will be the pirate king and joining him is the only way to achieve his dream, Law's crew would give their blessing just like the Franky Family and the Sun pirates. I just can't see Law calling himself Luffy's rival anymore if he really does 1v1 Kaido. At a certain point it's just delusional.
I’m not sure why you think that Robin’s dream entails perpetuating the status quo and keeping the truth about the Will of D and the Void Century hidden from the world. That’s the exact opposite message you should take from Clover and the others being executed in Ohara. Everyone in the world is going to know the truths of Laugh Tale, Robin’s not going to complete her life’s work and just... not tell anyone about it.
But for Law specifically, he can simply come along with them to Laugh Tale if he wants. It’s not that big a deal, Law’s like Vivi in that Luffy and the others consider him a member of their crew (not just friends, but actual crewmate). He’s just tsundere about it and he’ll never accept being an actual Strawhat, but the bonds between Law and the crew are strong enough that if he was to ask to come along so he can see the truth, without being a Strawhat, then he will. Oden, after all, was only “on loan“ to the Roger Pirates from Whitebeard.