For your son's 4th birthday, I advise you to take him to Chuck E Cheese. Rainforest Cafe may be "kid friendly", but that doesn't mean they especially cater to children with bells and whistles for their birthdays. They are certainly under no obligation to provide free gifts and cake.

It's not the restaurant's responsibility to keep track of a toy your kid left there. Sorry. And as for the food, as others have said, the time to complain about that was when you were there.