It’s happening. My two babies are almost lockstep in their development, despite a 17.5-month age difference. I knew they would overlap, and it’s been fun to watch the range of differences grow slimmer and slimmer.

At other times, my heart aches because I know the outcome here. A window in time will open, and at that moment, both children will be at exactly the same place, for just a blink.

In my mind, the heavens will open and a choir will sing, as the moment ends sharply and Mary Emma continues into adolescence, at a faster clip than Charlie.

I'm not diminishing his possibilities, by any means. I'm trying to come to grips with reality.

Overlapping stages

Both share bright blue eyes that twinkle when they're happy — or about to embark on some naughtiness.

Charlie is 2-1/2 years old. Mary Emma is one.

Charlie is knee-deep in Terrible Two's temper tantrums. He also has Down syndrome and subsequent developmental delays. Mary Emma has a remarkable knack for mimicry and thus has shared several tantrums, herself. She also has the sweetest, roundest face you've ever seen.

Both share bright blue eyes that twinkle when they're happy — or about to embark on some naughtiness. (They get the color from their mother and the twinkle from their father.)

Pride and pain

Recently, when the four of us went to eat at our favorite breakfast place, The Husband and I swapped kids midway through the trip. I walked in with Charlie and walked out with Mary Emma.

The contrast startled me.

I just want Charlie to be able to do anything.

Going in, Charlie worked so hard for every step, going from the car to the restaurant door. He lost his balance once but was holding Mommy's hand like a good boy, so I was able to quickly pull him back up. He grinned the whole way, leading with his belly and tossing out a left leg that refuses to admit it has a bendable knee.

On the way out, I clasped Mary Emma's hand and found myself hustling to keep up with her, as her toddling became a toddle-sprint combo. She wobbled along so quickly, she fell briefly but was back up before my reflexes could pull her up myself.

I felt equal parts pride and pain. I'm so proud of both my children, but seeing the baby literally run past her older brother brings an ever-so-slight twang to my heart. I just want Charlie to be able to do anything.

Sibling rivalry keeps pace

Mary Emma adores her big brother and wants to do whatever he's doing. Charlie is learning to play nicely with his little sister, prompted by a consistent refrain of, "Be nice, Charlie!"

He has his own agenda as he plays, and when she interrupts his Little People storyline, he screeches in frustration and pushes her aside.

I thought I'd dread the day the roles reversed, but it's been refreshing to see my daughter stand her ground when her brother loses patience.

Embracing the moment

I want her to understand what it means from me, not children on the playground.

I know someday, I'll sit with her and talk about what it means for Charlie to have that one little extra chromosome. I hope I recognize the right timing. I want her to understand what it means from me, not children on the playground.

But until that day comes, I will enjoy the innocence (and volume) of sibling rivalry and Mary Emma's struggle to keep up with her brother.

Because I can hear the choir warming up, and I can feel that window closing.

More about parenting a child with special needs

Having a sibling with Down syndrome
The truth about my child with Down syndrome
When strangers stare at children with special needs