A classic riddle in Chinese!
Here’s the answer:
The two big and two small can indeed sit on a boat drifting away (可以乘坐一艘船漂流). The explanation is that “two big” refers to adults, while “two small” means children. So, in this case, an adult and their three kids can all fit on one boat and drift away together!
I hope that gives you the answer you were looking for!