I might cynically put "quick sentence the kid SUPPOSEDLY wrote," because anyone who has visited a Mormon family ward has seen the phenomenon of kids bearing "their" testimonies.
But in this case, I'm fairly certain the kids did come up with their sentences. How do I know this?
Well, because amidst all the "Jesus loves me" and "My family can be together forever" and "Heavenly Father is happy when I keep the commandments" there were these two gems:
From the budding little scientist:
And from the kid who is probably exactly like the daughter I'll have someday:
Yeah you do, sweetie. Yeah, you do.