You stopped at one cookie? I'd count that as a success!

Don't let it get you down, if you didn't go back and finish off the pack when the kids finally got distracted enough for you to do anything. Yeah, you ate 1 cookie and stopped. Count it as your sweet treat for today and get rid of the cookies! Or put them way up on the top shelf that you have to get a step stool to get to. Out of sight-out of mind.
It's not a cheat, it's an improperly planned indulgance.
But seriously, if you feel cravings coming back and do want to eat everything in sight, then it might be useful to stay on phase 1 for a few extra days, but if it didn't set off any cravings you'll be ok.
Sarah