Tama situation has never been consistent of her goal about wanting set sail to go on an adventure. Yamato on other hand has been telling us throughout the raid that she wants to go on an adventure and see the world multiple times Tama on the other hand has only said it twice. In both the flashbacks and in the present story Ace and Luffy promise tama once they leave this place behind, she can live in a country where she can eat as much as she wants. Tama in her flashback when heading to onigashima she even says Speed she is tired of living in a country where she eats nothing but mochi and drinking dirty water. She sick of living under Kaido's and orochi's rule. In fact, she wants to live in a country where does see Momo as the shogun. She does restate what both Luffy and Ace said to her. After the raid was over does tama invite herself to be part of the crew? No, she ends up getting an adopt mother that will take care of her. It kind obvious that tama conclusion will not be for her to set sail with the rest strawhats at the end of wano.
I do not know how tama (in my imagination) in the future of story how would she be helpful to strawahts against the bigger threats. Such as the blackbeard pirates, the world government, the marines, and shanks (even though I know for fact shanks will not hurt child). Her life would be constant danger against the many enemies the strawhats would face. They would have baby sit her ass which would have strawhats not to cause ruckus or a steer throughout the island. Which is very unlikely the strawhats themselves would sit oddly by do nothing if their new friend they made on the island was in danger. The last time Luffy suggested of taking the children on his ship was during punk hazard. We know the Robin and Franky just said it would be bad idea. What makes situation any different that scenario? Eventually towards the end of that arc nami entrusted tashigi to look after the children in punk hazard. Who is say nami will entrust speed to tama?
The thing about Yamato on other hand there is mystery that surround her character and future development within the story. Yamato about her race, her mother, why does father refer her as a son, and where was she born. Yamato also future development that could lead to a sense self-discovery her own purpose in life. Her joining the strawhats levels the plain field moving the story forward against the bigger threats that are be going difficult for the strawhats overcome. Since robin is going to be target from world government and to many other pirates’ crews who might want her knowledge, where laugh tale is she is going need stronger protection from members of her crew. Instead of child who use summoning monster back her up.
If strong enemy grabs a hold of tama, she is good as dead. Do you really think it is clever idea to bring a child alongside on their voyage against the many enemies there going to face? Like zoro said, to Luffy back in water seven there not pretending pirates this is not game. Right now, Luffy consider one of the newest emperors of the sea. Knowing that is the case the government going think of them as a major threat going forward. They might send a buster call, vice admirals; and admirals just stop the progress. Two of the remaining emperors of the sea will clash with them. After the one piece is found the world government is going to be their biggest obstacle going forward. Knowing akainu sense of justice he does not spare children.