Neil Patrick Harris' adorable kids are growing up fast!

The Series of Unfortunate Events star and his husband, David Burtka, are celebrating some major moments in their kids' lives -- including losing their baby teeth!

Burtka and Harris took to Instagram on Sunday to share some cute snapshots of their 6-year-old twins, daughter Harper and son Gideon, preparing for Easter festivities.

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"Getting ready for Easter! Rice crispy treat nests with mini eggs," Burtka captioned a photo of his kids hanging out on the kitchen counter preparing some Easter-themed baked treats.

"Oh, and a missing front tooth!" he added, referring to the gap in Harper's beaming smile.

WATCH: Neil Patrick Harris Shares Adorable Family Vacation Pics With Kids Harper and Gideon

On Tuesday, Harris shared a similar photo -- snapped during the same day of baking fun -- in honor of National Siblings Day (only he was a little late).

"Happy #dayafternationalsiblingsday to my favorite two twins," Harris wrote in the caption to the pic, which he credited his husband for snapping.

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Later on Monday, Harris shared another adorable photo of his kids from the top of the Empire State Building, as they smiled for the camera in front of New York City's iconic, sweeping skyline.

"We didn't want to pass over this beautiful day in NYC without taking in the sights from the top of the @EmpireStateBldg. #grateful," Harris wrote.

WATCH: Neil Patrick Harris' Twins Fit Right in With the New York Art Scene

Last week, Harris and Burtka celebrated their 13th anniversary and the couple commemorated the occasion with some super sweet messages they shared online. Check out the video below for a look at how the pair celebrated their special day.