see [MENTION=152773]noglider[/MENTION] 's response on the bikes you named. to stay under $800 is going to be tough unless you're looking at at seat or a trailer.

xtracycle also makes an attachment for the bike if you already have it, but that's upwards of $600. it used to be called the freeradical but now it is called the leap. i don't have any experience with it.

best of luck OP!