I use glass gems for weight and stability in some of my patterns. Glass won’t rust or corrode over time, so it is safe to put inside your yarn, but I would not recommend it inside of anything meant for a child under the age of three.
I often recommend adding glass gems to the feet of dragons, etc. This will give them a little bit of weight, and glass gems have a flat side, so if you point the flat side down it also gives the animal stability.
You can find glass gems at any big box craft store or online. I didn’t even know what they were called when I first started searching for them, but I’d seen them and played with them before. They can regularly be found in the bottom of wedding center pieces and you’ll often find them in the floral section of your craft store. You can use other types of weights (people have told me they’ve used metal nuts, pennies, etc to do the same thing), but I prefer the glass gems.