Why does the fact that the Hubermanns have a child concern Max?

Max is concerned because kids can't keep secrets.... he's worried about getting caught.

Walter nodded. “He’s good. He still plays that accordion your mother told you about—your father’s. He’s not a member of the party. He gave me money.” At this stage, Hans Hubermann was only a list. “He’s fairly poor, he’s married, and there’s a kid.”
This sparked Max’s attention even further. “How old?”
“Ten. You can’t have everything.”
“Yes. Kids have big mouths.”