Columbus Day weekend in Portland, ME with kids
Glad you're coming! The Children's Museum (right next door to the excellent Portland Museum of Art) is very good: lots of hands-on fun: I have a 4.5 YO grandson, so I know! The Narrow Gauge Railroad down at the end of Commercial St. is also a good kid destination. Finally, for food in the same neighborhood, try Flatbread Pizza (next to the ferry terminal) for excellent wood-fired pizza (and the kids can stand next to the oven), local/natural ingredients, and a great view of the boats coming in and out.
Those are good suggestions. Duckfat is a good lunch spot for kids (and anyone else); they have PB & Js and fries and shakes (but waits can be long so go before they get cranky). Or take a ferry to Peaks, rent bikes (the bike rental place has trailers) and bike around the island, then eat lunch at the inn or the Cockeyed Gull, and get ice cream before you get back on the boat.