A father takes his son and daughter on a trip. There is a river on the way, and there is an empty boat at the ferry, which can carry up to 50 kilograms. The father weighs 50 kilograms, and the son and daughter weigh 25 kilograms each. How can they cross the river safely? Please describe the process of crossing the river in words.
爸爸带着儿子和女儿外出旅游，途中有一条河，渡口有一只空船，最多能载50千克，而爸爸重50千克，儿子和女儿各重25千克，他们怎么样才能安全渡过河去？请用文字说明过河的过程．
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