A children's book is colorful and has large letters. At the same time, when we grow up, letter size is reduced by age, and in old age there is much less color and letters are larger. So, if a page is for adults and for seniors, how do you design for better communication? How can we achieve this in UX Design ...?
It's important to remember that regardless of the targeted demographic, simplicity in UX is valuable. You're correct in saying that designing for children generally involves brighter colors and larger text, but there's no rule against doing the same thing for adults as long as it doesn't look unprofessional.
My advice would be to find a balance between child-friendly and adult-friendly that focuses on simplicity without being "offensively simple" or childlike.