With "Founders Day," it's an adjective. With "Parents' Day," it's a possessive. This makes sense, as the day is because of the founders, but for parents.
However, the word "children" is already a plural, so saying "Childrens Hospital" would be akin to saying "Founderss Day"...it's not the correct adjectival form.
And now that I have written my first two paragraphs... I see that HKMk23 has basically already said the same things I did. :-)
I applaud the attempt to encourage FReepers to check their signs more carefully. There are websites that poke fun at signs held by conservatives, and though we all make mistakes, it would be good to minimize it. Self-improvement is one thing that separates us from the Lefties...when faced with criticism, Lefties tend(ed) to be the ones to reply with excuses or defensive counter-attacks. It saddens me if we are to go that path.
And yes, I do use lots of ellipses and start sentences--even paragraphs--with conjunctions! ;-)
ObPunctuation: "This is dedicated to my parents, Jesus and Mary." --Thomas Eleri
"Just say YES to Oxford commas!" --Gondring
Idea for a FReep sign: "The following display of petulance and whining was made possible by the sacrifice of brave men and women in our military."