The rule of thumb used to be that if it fits in a 35mm film canister, it's too small to have around children younger than 3. Nowadays I suppose a lot of people don't know what a film 35 mm canister is...
In Sweden all infant parents are offered the "smådelscylinder" (literally "cylinder for small parts") [1] in which you can put things to determine if they're safe or not. Very handy.
