For sand opt for course grade used in concrete. avoid fine masonry sand. Pea gravel is one size like peas, about 3/8 inch or so in diameter. By raking out droppings, it means any best method to remove them from where they are. Some peeps use a kitty litter scoop to remove the gumdrops. Many...
If I was in your situation, I would opt for sand and pea gravel on ground. The other factor to consider is the pond. I assume you will be using a plastic type kiddie pool or plastic liner of some sort. It will get soiled frequently, so you will need to change water in it as needed. If...