This advertisement is from the June 1952 issue of Woman's Day magazine, which had a cover price of just seven cents at the time.
The copy for Royal gelatin desserts reads: "Doubly Ideal for Children! 21 Child Specialists Agree that Royal Gelatin Desserts are Ideal for Digestibility and Ideal for Food-Energy!"
Royal, a gelatin dessert competitor of Jell-O and others, has been around since 1925.
Here's a link to an interesting 1950 box of Royal with a Warren Spahn card on the back side. Royal had many different series of images on its boxes over the years, including sports stars, movie stars and Disney tie-ins. Some additional images of those boxes can be found here.