Joe,
I weigh about 220 and my daughter in the picture about 33 lbs. I do not think the weight will be an issue for you, however, depending upon how tall your daughter is, and where she sits, legroom might be a bit tight. I had my daughter sitting forward of the cross member, and she was quite comfortable, but she's only 3. She could have sat behind the cross member on a folding chair (I use folding stadium seats I got at a garage sale for 50 cents each) and had tons of room facing forward. Stability was just fine I thought, I haven't tried standing yet though. As noted by Gordy above, I agree that the next size up would be more comfortable for two.
It only takes 2 sheets of 4mm plywood, but there are a couple of tricky parts as with standard sized sheets you have to add little corners from the scrap to get to the right size for the butterfly shape. Adds a day depending on how fast your resin sets (about an hour's cutting and fitting work), but not too hard. I found bending the sides into shape tricky, and did not screw a piece of plywood on as suggested by the designer, so I heard a loud crack and ended up with a split I had to fix. I then put a small square of glass each side of the remaining undamaged 3 cuts and had no problem. That really was a problem to fix the split. I would definitely reinforce that joint as described if I did another build. The design is really economical on plywood and has nice lines, and paddles great.
I estimate it took me around 50 hours labor time to do, plus waiting for resin to set, but my paint finish was more utility than cabinet grade. I did it in around 3 months very part time.
Rich