Go bigger if your pack will accommodate it. You don't have to fill it all of the way if you don't want but it's nice to have extra water, especially with kids, plus the extra weight just adds to the exercise factor. I'm still using some old Camelbak bladders with the round caps; one of them has a small patch (Flexseal tape is one of the few things I've found that will stick to them) but they still work just fine. Nothing but water, rinse and sling excess out then put one of the expanding things in it to hold it open so it can dry. Wash periodically with dish soap or one of the Camelbak tabs or denture cleaner tabs, rinse well and air out. Easy.