What's the hardest part about being a parent?

As much as I want them to grow up happily and lead their own life, I don't want them to grow up. Now that my daughter is 7 years old, I find myself missing her baby/toddler stage despite having a 6 month old son. She's learning so much everyday, she even walks to school by herself (we live literally right next to the school), she helps me with her brother and has her own opinions on clothes and hair. One day I have to let her go, and I know I will scrutinized (lol) her boyfriends in case any of them are her potential husband. Same goes for my son too. Letting them go and letting them make their own decisions will be tough. My dad had it bad and still is finding it difficult to let me go.