Babysitting

Lavender gets to babysit Rainbow Dash and Soarin's children because she's the eldest of the main 6's kids and her mom thinks she could do with learning some responsibility. Of course Lavender is less than thrilled because grumpy teenage phase and because she knows the Soarindash kids are quite the rascals.
The kids themselves meanwhile are less than thrilled because they hardly ever listen to anyone and they don't need any dumb old babysitter.
So after some back and forth between the two parties, the kids decide to pull a Calvin and Hobbes on Lavender and lock her in a closet so they can do whatever they want.
All is going well...until baby Snowburst begins to cry. Since his two elder siblings have no idea what to do, they start to panic and eventually let Lavender out of the closet again. Lavender is ready to scold them, but when she sees their genuine upset, she softens up and makes a bottle of milk for the baby instead.
Both parties agree to a "I won't tell if you won't" deal and the babysitting goes much better after that. Quite often, in fact.

----------
Related headcanon: Pegasi families with newborns tend to put their houses on the ground until the baby has grown up enough to learn to fly.