Only thing I'd add here is you didn't tell us the age of your kid. In other words, until you child can support himself/herself, you're limited to a bike-mounted seat or one of the enclosed trailers. You have to stick with that until they're developed enough to use a trailer that has a conventional bike seat (usually around 2). Our youngest is 19 months old and is quite tall for his age, yet he's not quite to the stage where we can move from seat to trailer. Trailers do have some drag, especially since many mount them on a hybrid that's already heavy, but you get used to that.
This is another one of those areas where I don't think it's a bad idea to buy from a local bike shop if you're not really sure of what you want. You'll pay a bit more, but the advice you get may stop you from making a poor selection. I'm all for buying online when it makes sense, but for a specialized product such as this, buying the wrong thing could be an expensive and annoying goof.