I've bought from these guys before. There are also other sellers on Ebay. 1" conduit is the best choice for strength.
https://www.ysbw.com/Canopy-Fittings-1-Inch-s/7.htm
First step would be how to reinforce the center seat to be strong enough to support the foot pads. boat seat tops are very thin and soft, so you need to beef up the area you will mount these to. When the canopy poles are mounted it's a very long lever, so it needs to be strong.
https://www.ysbw.com/Canopy-Fitting-1-Foot-Pad-p/2255.htm
This could be as simple as adding a 3/4" plywood topper to the seat and then reinforcing that with angle brackets to the side of the boat. Not sure about your abilities in that regard. Do you understand what I'm describing ? Save the photo above to your computer and then blow up the area where the footpads are.
If you are most concerned with shade and less about fishing, it's also very easy to just make something like this.
If you make it like this you can use very lightweight 3/4" tube and fittings. You also would be much less worried about reinforcing seat tops. Let me know what works best for you and I'll talk you through how to do it.
If you just wanted to make a shaded area for your child you could also consider something as simple as this: