I don't think switching diapers is going to help you out. Adding a doubler / booster might. Or going with AIO or PUL/nylon covers.
The 28 month old you could introduce to the idea of using the potty, maybe the 15 month old too. It sounds like they may be waiting for a clean diaper to pee in before they pee. Maybe take off the wet diaper and let them sit on the small potty for a minute or two before putting a new diaper on ? Just a thought - if they pee, great. If they don't, at least they're becoming comfortable with the potty without the pressure to perform.
The 28 month old you could introduce to the idea of using the potty, maybe the 15 month old too. It sounds like they may be waiting for a clean diaper to pee in before they pee. Maybe take off the wet diaper and let them sit on the small potty for a minute or two before putting a new diaper on ? Just a thought - if they pee, great. If they don't, at least they're becoming comfortable with the potty without the pressure to perform.