Raise the camera just above eye level so it angles down on the group slightly. This shift visually lengthens and slims bodies. Also by forcing the group to lift their heads slightly to be eye-to-lens hides their double chins.
Posing tips
Don't ask them to say cheese. Instead, just before shooting, ask them to sing Old Macdonald Had a Farm. Kids know the words and respond well, and even the adults may laugh. You get natural body language and better facial expressions.
Use a symmetrical shape of grouping. Try an inverted triangle with one person in the front middle, place pairs of people beside and behind, three in the next row etc. Photo may look more balanced if everyone gets the same amount of space and each person is showcased equally.