Be honest but don't give too much info - they're little and they won't understand anyway! I told my boys that I had to have a lump cut out of my boobie but that the hospital was going to make it all better again. I actually had a mastectomy and TRAM reconstruction so it took a while to get better! They knew to be gentle with me and that I couldn't do the sort of things with them that I used to for a little while. I was actually amazed how gentle and loving they were!
Good luck and we are here for you,
Louise x