In general, we don't recommend installing bike child seats on our bikes. I've researched this topic quite a bit, and while there is some disagreement, I feel that trailers are safer. They're much more stable, have a roll-cage to help protect your child, and they don't affect the balance of the bike as much as a seat installed on the rack. A fall from 4-5 feet off the ground, in case of a crash, is quite a strain on the neck of your child. Ultimately, this is something you'll need to decide for yourself, but that's my $.02 worth. I encourage you to do some reading on this topic as well, so you understand the pros and cons.
EVELO bicycles can easily tow a trailer so that you can take 2 children with you. There are many brands on the market, at different price levels, and most will attach to any of our bikes. The specific thing to look for is a trailer that attaches to the rear axle of the bike, and not the frame. Here are a couple of examples: