All aboard the birthday train.

Maralee Nichols recently celebrated her and Tristan Thompson's son Theo turning 2 years old with a train-themed birthday bash. In a series of photos shared to her Instagram Stories Dec. 4, Maralee and Theo are seen coordinating for the party with matching outfits consisting of denim overalls, red bandannas and newsboy-style caps.

In one pic, the mother-son duo posed near red and blue balloons that spelled out, "Chugga Chugga Two Two." Meanwhile, in another, Maralee cuddled with a large white teddy bear that sported her son's first name on its hat.

But that's not all, as the model also shared several snaps of her and her friends dressed in theme, as well as a look at Theo playing with toys that included building blocks sporting his moniker.

This, of course, isn't the first time that Maralee, who previously filed a paternity suit against Tristan—also dad to True, 5, and Tatum, 16 months, with ex Khloe Kardashian and son Prince, 7, with ex Jordan Craig—has shared glimpses at her family life.

In fact, in honor of Theo's first birthday, Maralee reflected on her first year of motherhood.

"I can not believe you are already one," she captioned a December 2022 Instagram post of the pair. "Spending the past 365 days with you have been the best days of my life. You are the sweetest little boy, your smile brightens up any room."

She continued, "You are my greatest blessing, my world, my heart in human form. God knew I needed you. I love you more than anything."

Keep reading for a look at Maralee and Theo's cutest pics.

Theo showcases his athletic skills in October 2023.

Maralee shared this photo of herself cuddling her son on her Instagram Stories on July 29, 2023.

On July 14, 2023, Maralee shared this photo from her and Theo's trip to the San Diego Zoo.

Theo meets an elephant.

Theo read a book before bed in September 2023.

Maralee pushes her son on a swing.

Theo checks out his surroundings on a day out with mom.

This boy holds the key to his mama's heart.

Maralee Nichols documents her son Theo Thompson enjoying a Southern California sunset. 

While exploring an aquarium, Maralee revealed Theo's "favorite words are 'wooow' and 'coool.'" 

It's a magical mother-son date at the Disneyland Resort in Anaheim, Calif. 

Maralee celebrates her first Halloween as a mom alongside baby Theo. 

"October with my pumpkin," Maralee shared on Instagram. 

You never know who you will find at the pumpkin patch! 

"Can't believe my baby is 6 months old," Maralee wrote from the San Diego Zoo in June 2022. "Theo's first time at the zoo, he loved looking at all the animals."