I'm going to buck the crowd here and say I hate trailers and much prefer to have the kids on the bike with me. I don't like that the kids are down low, less visible to cars, less audible to you and, with 2 in the trailer, fighting between them is inevitable. It's awesome for them to be up on the bike with you to be able to talk to you and see their surroundings. I will say that if you are going to have both front and back seats, it's much better if you have a step-through frame bike. They're available on my local craigslist in abundance (seems there are lots of people who buy women's bikes, barely ride them, and then resell them). Ideally, it's best to have swept back handlebars and a long effective top tube to give you more space to mount and dismount. I also recommend a wheel stabilizer (available through Velo Orange) to help control the front wheel and a double kickstand (the M-wave double kickstand is inexpensive and functional). Regarding the seats, for an inexpensive rear seat that's widely available used, we really liked the Topeak Babyseat. If you want a rear seat that will also allow you to have panniers, I would look at the Yepp Easyfit with the XL adapter and put it on the Axiom Journey rack. Front seats are harder. I've tried many of them at different points. The WeeRide is hard to fit to bikes, creates an artificially high top tube, and is quickly outgrown. The iBert is nice and relatively inexpensive, fits all bikes, but also quickly outgrown (my kids outgrew it by length far before they outgrew it by weight. I personally didn't like the Yepp Mini. I felt like the angle of the seat and high back really cramped the mounting and dismounting space and there was an uncomfortable hard metal bar on the bottom that I hit while pedaling. I love the Bobike Mini, which is what I have now. My 3-year old fits in it well still. But unless you can find one used, it's rather expensive.