jmc

The sum of the ages of the fifth graders is $33\cdot 11,$ while the sum of the ages of the parents is $55\cdot 33.$ Therefore, the sum of all their ages is $$33\cdot 11+55\cdot 33=33(11+55)=33\cdot 66.$$Since there are $33+55=88$ people in total, their average age is $$\frac{33\cdot 66}{88}=\frac{33\cdot 3}{4}=\frac{99}{4}=\boxed{24.75}.$$