I actually really like how you've drawn the littlefoot. The character really comes out.
The safest bet would be to email Adam Phillips himself and ask, otherwise if you're not aware of any "DO NOT DRAW" then it should be okay. In a way I think it unlikely that the creator would actually mind since as long as properly credited it means exposure!
thank you very much. feel free to ask me to draw anything you want me to,i might consider to if the idea is good. but i'm still a rookie,and i use a mouse.