smc

The sum of the ages of the fifth graders is $33\ast 11$, while the sum of the ages of the parents is $55\ast 33$. Therefore, the total sum of all their ages must be $2178$, and given $33+55=88$ people in total, their average age is $\frac{2178}{88}=\frac{99}{4}=\boxed{24.75}$.