It's a major no-no for kids to be sat in any exit row (includes all rows where there are main doors as well as emergency exits - centre section on widebodies excluded, btw - endangers the lives of those on board and is covered under bothe JAA and FAA rules. If you see something like that, you should IMMEDIATELY complain to cabin crew and demand that they be moved. If they refuse, take their name and report them to their airline and aviation authority.
Primary reason for this? Parents are more likely to be concerned about their kids than getting exit open for other pax. Logical, really. Pax sitting in exit rows are required to be over 16, fit, healthy, able to read and hear instructions, and have no dependents.
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Happiness is a warm L1011