You have a pretty good start on your photography gear. Since you are diving into shooting portraits, the very next thing I would purchase is a collapsible reflector. A medium to large size reflector would be a perfect (and affordable) piece of gear that would allow you to start introducing fill light into your portraits. You could grab a new one on B&H Photo (I have one similar to this and it works great!) or pick one up on eBay. Using a reflector to add fill light will make your portraits pop! The only downside to reflectors is that you usually need a second person to hold them while you shoot (maybe bring one of your kids along as a helper!).
The next piece of equipment would be to add one flash head (550EX or 580 EX). Further along, I suggest getting at least one more so that you can be even more creative. The 550EX is no longer made, but they are solid strobes. You can get them on eBay for a very reasonable price. Adding a couple of flash heads to your gear bag is important as you start taking more and more portraits. I know Canon flash heads are expensive - but it's something to shoot for!