I always respond by saying " professionally I try to refrain
From sitting on children, it tends to be bad for business.".
I actually have a tshirt that says something like it. I bought it from THE ooey goey Lady... You can check out her website..
I love it!!! Now...if I can actually remember it when I need it (I'm notoriously bad at messing up jokes too!).
Children are great imitators.
So give them something great to imitate.