Let me get this out of the way - I hate An American Tail (the original) - I think it is a gloomy, morose and overly sentimental piece of tripe. What's more, you try getting a child to watch this trek into unhappiness and gooey emotions... it's not going to happen! Fievel Goes West on the other hand is a much lighter affair, with more cheer and one or two really lovable characters the kids will want to watch and adults won't want to smear the screen with chocolate because of. It is a little two dimensional (no pun intended) and it is also a little episodic and disjointed in places, but it recovers well and bounces back for a great finale. In short, if the kids have been good then let them watch Fievel Goes West (if they've been naughty then you could always punish them by making them watch An American Tail). I have always enjoyed this film and it continues to lighten the day.
