I roll nori with a bit of brown rice and my DS (almost 2) will eat it like that. Of course, he'd rather I not roll it in rice.
Agar-agar is my favorite seaweed. You can use it to make vegan jello (kanten). So yummy and good for you! The recipe should be on the package and online. I'm going to make some tangerine kanten this weekend.
When I make beans, I cook them with kombu (another seaweed). This is a great way if your family isn't ready to embrace seaweeds and it helps make them more digestible. Just soak for 10 minutes first, dice and then place with the beans.
We also make miso soup with wakame (yet another one) and I just serve the broth to my DS. He loves it!
Also, be careful how much seaweed you give your child. The younger they are, the less they should have. I only give my DS a total of 2 sheets of nori per week at the max.
Good luck and yummy eating!