K.V.
My son was 1-1/2 when we went last year - we go to the zoo frequently and he did enjoy it, although he was really unsure of what was actually going on there. :) I'm sure we'll participate again this year.
Another fun Halloween thing (might be more exciting for your older child than your younger) is to do the Halloween thing at Crossroads Village Huckleberry Railroad. They have trick or treating in the village and also train & boat rides. Their train ride is 30-40 minutes long.... My son loves trains (he's 2-1/2) and enjoys going here, too. (Not just for Halloween, but throughout the summer.)