Where is Collin Gosselin? Kate Gosselin opens up about his absence from sextuplets' birthday party

Kate Gosselin celebrated her sextuplets 13th birthday on Monday's all new episode of Kate Plus 8, but one of the reality star's children was noticeably missing from the festivities -- her son, Collin.

Five of Kate's sextuplets -- Aaden, Alexis, Hannah, Leah and Joel -- had a great time at their camping-themed party, which was filmed back in May, and included fun activities and tents set up in their back yard.

As part of their celebration, the boys foraged for supplies that their mom had hidden around the property, and went on a scavenger hunt while learning how to use a compass. They also cooked their own food -- like corn and hot dogs -- over a camp fire.

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Meanwhile, the girls decided to have fun "glamping." They made flower crowns with arts and crafts kits and got mani-pedis from an on-site manicurist. Their fancy dinner was filet mignon served on a table, just like how elegant people camp.

Finally, after all the fun and games, it was time for their cakes. That's right, two awesome cakes to commemorate the milestone birthday. For the boys, there was a cake decorated with fire, while the girls enjoyed a regal, two-tiered pink cake, complete with a pink tent on top and the number 13.

While a good time was had by all at the party, the kids' 13-year-old sibling, Collin, was nowhere to be seen.

Kate revealed in August that she had enrolled Collin in in a program away from home to help him cope with "special needs."

"[There's] a fairly fluid diagnosis of what those needs are, but he needs to learn certain strategies to help him deal with things," the 42-year-old reality star told People Magazine last year. "This has been a struggle we've had for a very long time."

Kate said she and her seven other children miss Collin all time, explaining, " it's been hard because there is a huge hole in our family without him here... But it comforts us to know he's where he needs to be right now, and I can feel good about that."

See photos of Kate Gosselin through the years:

On Monday's episode, Kate got emotional as she reflected on her son's absence after the family sang "Happy Birthday" and snapped family pictures next to the cakes.

"The photos around the cake, I have them for every year, every child, every birthday. We always have the big birthday party at home on their birthday, [I] got 'em for every year," Kate shared. "This year [was] much like last year, obviously the difference is that Collin wasn't there."

"I am comforted with the fact that my kids, all of them, each unique child, is receiving exactly what they need and that hasn't changed," she continued. "I've said that before, I'll say it again. It's a bittersweet moment [and] we've had many of them. You can't do anything without realizing he's missing, so we may not say it, but it's always there."

For more from Kate on her difficult decision to enroll her son in a special needs program, watch the video below.