Forget about brands or packages. As with adults, fit is the main criterion when selecting children's snorkelling gear. In the case of fins, check that the foot pockets of any pairs you consider match their feet dimensions not only length-wise, but also in width terms too. Children have growing feet, so don't expect the same fins to fit a year or two down the line. Don't choose fins with colourful, sparkly but unyieldingly stiff plastic blades which may damage young tendons. Soft, flexible all-rubber fins have the advantage of being perfectly suitable not only for simple snorkelling - that's what every snorkeller used in the mid twentieth century after all - but also for lap swimming practice. They're available at very reasonable prices from swimming goods stores.
As for masks, fit is essential here too. Let them try different models on in the store, applying the masks to their faces without attaching straps and breathing through their noses to create a partial vacuum. If the mask stays in place, it may be a reasonable fit, but you can never tell until you try the mask in the water. Avoid masks with PVC skirts, go for skirts made from rubber or silicone, which are also softer and more durable. A simple "J" shaped snorkel may suffice.
A number of years ago, I spent a pleasant hour or so snorkelling with others at La Jolla Cove in Southern California. The people wore every kind of fins, masks and snorkels imaginable. The common factor was that they were all having a great time, which is what snorkelling is all about, as this picture from the 1950s eloquently demonstrates:
