Nothing. I've had bad days, but you simply forget that. My mom was always like: It's no work at all. It's all fabulous and you kids are the sweetest ever. When I got small kids, I was surprised that it was quite some work, because my parents never complained and then all of a sudden my mom was like: oh it's so hard. Let me help you. LOL I thought you said it was all so easy.
The hardest was other ppl who demanded you keep 3 boys quiet, cause they had to have quietude. I wanted to gift em ear plugs. Just an extra load to make your life hard. The kids were great, but the adults were annoying at times. Once an old man in the train started to bark and I just said hey thanks for the help m8 and stepped out and waited for the next train.
And the heaviest was having to carry a bag everywhere while walking with drinks and food or more bags with boats and whatever and they'd love to walk right in front of your feet.