I’ve sat on this for weeks, and I have a proper answer finally wow. Sorry for the wait! Bowser’s development has been a little all over the place, since his character is a primary driving force to what goes on in the background of the narrative.
Bowser is a loving father, but he doesn’t stay that way. The Koopalings come into the picture before Bowser Jr. does. Nonetheless he treats the seven very well, almost like they were his own - or more however well he can with a kingdom to rule and attention heavily divided / generally invested elsewhere. His relationship with them is pretty varied across the eight Koopa children. He’s at his most affectionate with Bowser Jr., because he is of his blood and was involved with him from the very beginning. He dotes on him in a lot of ways - toys, attention, promises of greatness… Jr. is generally at the forefront of his mind, to a fault.
In regards to the rest, things get a little spotty. If there was a pecking order as to who got the most out of their reltaionship with Bowser it’d be Iggy and Lemmy at the bottom, Larry and Ludwig at the top, with Wendy, Morton, and Roy somewhere in the middle. Everyone’s relationship starts out on the same level, and either becomes closer or sours as time advances. A number of the Koopalings do eventually just become tools to Bowser, as most of them wouldn’t have developed into what they become without his direct influence. Bowser’s treatment of the Koopalings is more or less a reflection of sorts to the evolution of the conflict between the Koopa Kingdom, and the Mushroom Kingdom: things start with wanting to expand his reign, looking for allies, building bonds of trust and etc, and then take a turn to looking for weapons, tightening control, and exercising power. He does encourage all of the Koopalings to call him “father”, though whether or not that means anything to them depends on the individual.
Bowser’s treatment of the eight Koopa children lends to some other interpersonal conflicts amongst them. Ludwig in particular is wholy invested in the idea that whatever Bowser is doing for the greater good and champions it as such, while, say, Iggy isn’t so convinced. Larry wants to believe the former, but personal findings and experiences lean in a different direction.