Write children 3 and under free as an inequality

Children ≤ 3 (are free)

Step-by-step explanation:

This is strangely worded by I think this is what you were looking for.

The key take away is, if it says something and under, you know it means that it can either be exactly that "something" or anything that is less than it, so you would use the less than or equal to symbol.