When my oldest was 3, I took her fishing at Chester Frost park. I didn't know if she would like it or not, so I figured it would be best to take her somewhere she could play if she didn't like fishing yet, that way we could still make the best of our time together. On her first try, she caught a 3lb 1oz blue gill. I knew it was huge, and took a picture of her with it. ( i did not know 10 years ago that it broke the state record by 1oz.) However, she didn't care in the slightest about that fish, or any of the others caught that day. She just wanted to spend time with her daddy. After multiple trips between the lake and the playground, I admit, I was frustrated, I wanted to fish all day. I made sure, that no matter what, she had fun. It's a memory we'll carry with us the rest of our lives.
I've learned after three kids, (2 girls and 1 boy) taking them fishing at a young age is not about what you plan to do, but what they want to do. It's best not to confine them in a boat. I've had a lot more fun with them on the bank watching them fish, and explore, than I ever had taking them in the boat. If I do take my pole when we go, I make sure I use a bell or a bobber, that way my attention is on them and not what I'm doing. They'll test my nerves, and my patience, but they've never said they had a bad time, or they don't want to go again. And looking back on it now, those are, of all, my favorite fishing trips.