There is a movie on Netflix called Penguins Bloom. In the first 5 minutes of the movie Naomi Watts and her son are jumping on a trampoline. In the next scene he is laying on her stomach and you can see his hand in her armpit as she's laughing, kicking her feet and saying "Noooo" it's really brief but it's the only true tickle scene or confirmation that I know of involving Naomi Watts.