And they were left particularly struck at what a good brother Junior is to Harvey.
The 14-year-old not only looked more mature than ever, but showed incredible kindness to Harvey.
She enjoyed a day full of cake and party games with her five children - Harvey, Junior, Princess, Jett and Bunny.
Fans noted how during each party activity, Junior made a big effort to make sure Harvey was included.
Harvey, 18, has Prader-Willi syndrome, has autism and is legally blind.
The video, which already has earned over 140,000 views, has dozens of comments praising Junior.
One viewer wrote: "Junior is so sweet towards Harvey! He is so matured! :)"
A second user gushed: "Loved it. Your son is a good brother how he keeps Harv going."
A third commented: "Love that Junior is so nice to Harvey. He's so caring about him love it."
And a fourth posted: "The way junior helps Harvey out is so cute."
The video showed the family eating birthday sponge cake and playing various games - including Jenga and sack racing.
Other fans couldn't quite believe how mature and tall Junior now looks.
One viewer commented: "Omg Junior, I didn't recognise him, he has grown in height and maturity. He looks so different."
And a second viewer observed: "Wait! When did Junior grow up so fast??"
A third user praised: "What a young gentleman Junior is!
"He said to every one of his siblings 'what do you say?' Thank you! Bless him, lovely lad."
Katie's birthday comes just days before her eldest son Harvey's.
When he reaching the milestone of 18 on Wednesday (May 27), Junior also paid a sweet tribute to his half-sibling on Instagram.
He shared a series of photos of Harvey to his some 210,000 Instagram followers.
His sweet caption included: "Happy birthday big bro.
"I love you so much you don't understand. You're the world to me.
"I can't believe you're 18 today. It's muuaaadd. Love you so much and can't wait for more years to spend with you."
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