I don't know where to find the fish consistently, but I have 4 boys (and 7 girls). For a positive experience, call it a boat ride instead of a fishing trip. Then any fish will just be icing on the cake.
And take LOTS of snacks and drinks. And a toilet. Get them their own sunglasses (cheap, because they will inevitably be dropped in the lake). Pack jackets for them in case they get cold. Bring sunscreen and use it. Let them steer the boat. Let them lose some fish while netting and don't get mad.