Hi Michelle Janine,
Here's an interesting Canadian site on MRSA. It does say that after exposure to someone "colonized" with MRSA, 20 to 60% of the patients in the hospital do develop an infection.
Here's the site:
http://www.ammi.ca/pdf/MRSApositionpaper.pdf
So now, the question is, was the child still "colonized" with MRSA? Or had he completed a course of antibiotics such as Vancomycin and completely recovered?
So, for right now, rather than worry, I'd make sure I did things to boost my immune system, such as take extra Vitamin C and Echinacea, rest, and take plenty of fluids.
Also, as long as you used good handwashing technique, hopefully you got rid of most of the MRSA before you left their house.
If it is of any reassurance, I did know a nurse who got MRSA while working at the hospital where I am employed. She was treated with antibiotics and recovered very successfully.
I hope you were not exposed, and good luck.