Nah, ya gotta do it as they "Know this is the special thing they do with dad" so thats why they say it. Just like "lets throw the ball around" "lets play catch" you just do it no matter how tired you are as they want to spend time with you.
Remember, soon it will be no I don't want to do anything with you Dad. Or can I borrow the 914 again?
That's how my son got to like automechanics, by "helping"me. Start them off by teaching them the tools, what is socket, box end wrench and then have them hand it to you. Later on let them use them. Show them the steps to take something apart and explain why. Show them how to assemble stuff.
They will love you for this later as it teachs them how to think. Something they don't learn in school anymore.
Geoff