Last year my sister-in-law got married and she had an official photographer. I was asked to take candid photos the photographer wasn't getting (I had a T2i, 28-135mm and no ext flash). We talked the prior evening at the rehearsal dinner about my intentions and I said I would not be in her way. I was just going to roam. She was totally OK with this and said she might need my help at some point as her assistant wasn't going to make it. Sweet!
During the ceremony and important parts, I stayed in the back and around the edges. She had the gear, the know how and it was her job to get those important shots. If I had some better lenses, I could have captured some angles she forgot but oh well.
My youngest sister-in-law is getting married next year and asked me to be THE photographer. I declined, but I'm happy to do what I did above, especially now that I have my new 5Dmk3 and 24-70L mkI. =)
Edit: The shot of your son is great. If you shot RAW, you can probably bring up the shadows of your daughter a little. One thing people forget about is being paid to take the shot or just taking the shot. Candid photos can come out just as good or better sometimes than the ones that were heavily thought through.