How would you feel about a parent who has a young child in the back seat of their car while driving after having a lot to drink -- enough to put the parent well over the "legal limit" of alcohol in their blood to be driving? Do you think it would be right to take away the child from this parent because the parent has shown willful disregard of the child's safety?
I'm not sure I follow your analogy - in one instance there is willful endangerment in the other they probably came to the US illegally so their kid(s) could have a better life. If the only link you're making is the 'legality' or lack of it in both actions then that's a pretty weak argument. What is illegal isn't always morally wrong and vice versa. Maybe I'm misinterpreting your argument; if so then I apologize in advance.
Crossing the border illegally is not the same as going on a day hike. Hundreds of people die every year while crossing the border, most of which are not violence related. Further, ignoring the fact that crossing the border in itself is illegal, there is a lot of illegal activity on the border that puts anyone who crosses in harms way.
I would also dispute that parents are crossing with their kids for their kids to have a better life. The number of adults detained with children nearly match the number of children detained with adults, which indicates that most adults bring exactly one child across the border. I would argue this indicates that people are bringing one child with them in order to avoid getting detained by ICE.
One good example of the above happening is the girl who was recently featured on the cover of TIME magazine. According to news reports, according to the father, the girl's mother brought her daughter across the border illegally several years ago, the daughter was never separated from her mother, however, the mother left multiple other children behind when she left.