Final answer:
The playground age restriction 'You can only use this playground if you are 7 years old or less' is translated into the inequality a ≤ 7.
Step-by-step explanation:
The message 'You can only use this playground if you are 7 years old or less' can be summarized using a mathematical inequality, similar to the example given for children older than 12. When translating age restrictions into an inequality, you define a variable (like 'a') to represent age. Therefore, to summarize the given message with the variable 'a', you would write it as a ≤ 7, which means 'a is less than or equal to 7.'