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I think you'll struggle to get both a loose child and a child in a car seat on the front of any of the front-loading bikes. Car seats are bigger than they look! You might want to have the older one in a Hamax or similar until the younger one is big enough to ride without a dedicated seat. Leaves you space for picnics, cushions, shopping etc.
You are well into bakfiets territory. Do it!
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I've seen/heard of people using a Thule sling for little ones and slung from the bullitt canopy. It's made for jogging strollers but I guess if you take it easy (as you would with a little one anyway).
Thanks everyone for the tips. Not to familiar with the cargo bikes world so I was always bent on getting all the kids in the box on the front. Not ideal so might be looking into alternatives. Still have lots of trailer straps lying around but the other half will be riding the cargo bike more than me and she'll need some convincing :p