Horton Hears a Who is one of Seuss' best books! I'm 21 and I still enjoy it. It teaches a valuable lesson in helping those who cannot help themselves. It is a little intense at the end when animals attack the protagonist, so I'd say to wait until your kids are old enough to not panic!
Wonderful story with a great message, but can scare younger kids with Horton being tied up by the monkeys and them threatening to throw the clover into hot beezle nut oil.