Over Myself

In Sparring Partner, we're shown that Raph is nervous about hurting his brothers, and so he goes out of his way to find someone else who he can spar without fear of hurting. We're shown in Shadow of Evil that Splinter can spar with his sons and hold his own, but evidently, he chooses not to the majority of the time, even when they basically beg him. Not only that, but Raph wants to learn to be a hero, and '12Splinter is the perfect 'wise Master Sensei from tv' type of person to train him! Raph meeting a trainer who can take whatever he throws at him and teach him how to be a hero would be a dream come true for him.

Leo is shown in multiple episodes (like Jupiter Jim Ahoy and Air Turtle) to adopt father figures at the drop of a hat. All that's required is for them to be 1) cool and therefore idealizable and 2) pay him any attention at all. He folds and loses his senses almost immediately when 'father figures' come into play. He wants them to like him, so he'll go out of his way to be liked. Considering '12Splinter's natural aura and wicked fighting ability, Leo would not only cling to him in a heartbeat, but he'd go out of his way to show off and secure his undivided attention. 

Donnie has his *thing about parent-age adults and would see the aloof but welcoming '12Splinter as a challenge to earn approval from. His relationship with his dad seems to be a somewhat transactional relationship (Perceived in Smart Lair and The Ballad Of Rat Man) where he gets attention when he has something to give, so he wouldn't know what to do with Splinter's neutral take on any of his experiments/actions. He's not only a parent-aged adult, but he is his parent from an alternate dimension (technically) and therefore must be awed and enamored by Donnie's genius. Only problem- '12Splinter doesn't want anything from him and isn't overly impressed by futuristic gadgetry. Their interactions only spur him to be more forceful about "earning" his attention by any inventive means necessary.