mike, they're viruses, and with little spiderman and bro running around, interacting with other kids then they're volumetric exposure to these increases, and you get them brought home to you.
The symptoms of a cold (rhinovirus) are actually your immune system attacking it so aggressive symptoms could be said to indicate a stronger immune system, so it's unlikely you have the bad aids. What you need to do is therefore control the symptoms when they arrive, which simple paracetamol does perfectly well, eat and drink well during the colds and generally you'll accelerate the recovery.
Echinacia is toss, and a waste if time - most, if not all homeopathy remedies, if they work become classified as medicine. There have been sufficient clinical trials which have shown no beneficial effect to this.
Ones that have been show to assist in controlling the immune response to colds have been vitamin c and zinc. You should get enough of these in your daily diet without the need to supplement. If you are concerned i'd recommend a daily probiotic and good diet, and that's pretty much all you need for a tough immune system if you're exercising at least moderately.