What do you want to catch? Fresh or salt water? Are the kids already into fishing or just starting? Do you trust them near a river?
I'd start by thinking of a place that they'd like to go if fishing sucks, then try to find a fishing locale nearby. For example, if they like the trampoline park then go to Brownsville Bar then bail to Central City Fun Park when they get bored. Or go to the C/V and then visit Castle Fun Park when they get too cold (assuming it's open again after the flood). Deer Lake is next to Burnaby Village Museum, as well as a nice park and art gallery. The Fraser River Discovery Centre at the New West Quay is great, and there's some good food at the quay... the Brunette is nearby but takes some dedication (and waders) at this time of year. Hayward Lake is good for pikeminnow from shore but fishing may be slow this time of year and there's nothing very interesting nearby, other than the Powerhouse Museum and Bruce's Market.
Nothing jumps out to me as being great this time of year with multiple small kids... maybe try Silverbrook Trout Farm in Abbotsford and then hit somewhere sweet for lunch on the way back?