For example, all passengers with seq. numbers 1 - 59 come forward first, then 60 - 119 etc. but allowing exceptions for a small overlap for passengers in the same party, or at least certainly with children.
Haave seen it in the past with boarding with children (even with allocated seating) where you ask people to come forward and its a young child with mum and dad, then 2 teenagers with granddad and grandmas so for 1 child you have 8 or 9 boarding.
Sadly no matter what you do you will still have the feckless turning up at last minute or checking in last minute. Then demanding that everything else is rearranged to suit them because they wish to save a few quid, course that is not a worry when they hit the airport shops.
When travelling as a family we pretty much always paid for priority boarding because it then left you in control of what happens in getting a seat rather than relying on the goodwill of others.