I don't use any organizers on my bags, except for my giant Goyard totes. Things don't shift too much, but I mainly have large items. I put small items (lip balm, bandaids, etc.) in a smaller pouch.
As for the Lindy 30, I think that size was best for me proportionally, but it may not fit all I may need for kids. I didn't use the Lindy all that much, and when I did, it wasn't with kids (shopping trips, lunches). I usually wore it over the shoulder, and it was okay, but it has bulky proportions. I like hands free, so crossbody definitely for me! I even extend my K35 strap to crossbody so I can use for light child outings.
Hope that helped!