60 minutes or less in length, coherent around one message/theme
Any 'talk' or 'sermon' makes one point with a double audience in mind (children-and-adults) ... so all of congregation is engaged
Children who participate through reading, prayers etc previously rehearse their roles
Songs as a total mix offer participation to whole congregation in worship of God
Save for a moment or two, the congregation never become spectators - always remain participants in the act of worship
One outcome is a congregation looking forward to the next family or all age service
(My strong personal preference is for such services to be Communion services, not least because the act of receiving communion involves significant congregational participation through movement, but there are arguments for/against).