So Maxine and Donatello aren't really the couple who would want kids, but if they were put in the situation where they acquired a kid and absolutely had to take care of them, they would feel personally responsible for the child and would begrudingly take care of them and depending on the circumstances, eventually grow fond of the kid
What can I saw, I'm a sucker for "reluctant parent" kind of stories, like The Last of Us and whatnot
So in the event that Max and Donnie were in the right mindset, if they did acquire a kid, this is what I think they would look like, meet Galileo and Percival. Yes they're both named after astronomers because Maxine and Donatello are nerds.
They're both Donatello clones who Maxine and Donnie rescued from an illegal lab that was trying to make mutant super-soldiers by trying to clone mutants, for the most part it was a failure and Max and Don sabotaged it, but as they were leaving they heard baby cries and then found two successful clones of Donnie as babies who didn’t go down with the lab
They couldn’t really...exterminate the babies or leave them there, so they rescued them. They took care of them in secret from Donnie's and Max's family as they tried to figure out what they would actually do with the babies, but after a couple of months of raising the kids they thought "hey, this isn't so bad, maybe we can do this" and decided to give a try at parenting and adopted both of the kids officially (well, as officially as they can)
Galileo is a sweet bean who prefers gardening, painting and listening to music in his room much more than fighting or even dangerous chemicals exploding in a lab. Meanwhile Percival is an edgelord who'a going through his "it's not a phase, mom!" phase of wanting to be like his parents during their adventuring days, he's a secret vigilante and has a knack for making explosives and cracking skulls.