starlight - perhaps you've been lucky to have babies who just nod off when tired and who were happy on 45 minute naps? My 3-month DS needs help to fall and stay asleep - if he's tired but I don't help him then he goes from whingeing to crying to screaming uncontrollably. I don't do routines, he can sleep when he wants, but he still needs a bit of help. My DD was the same.
What I do is carry DS in a sling all day. When I can tell he needs to sleep, I need to jiggle around. Sometimes this is quick, sometimes takes ages - depends a bit on how loud DD (4) is being! His little eyes generally pop open at 45 mins but I give him another jiggle and he will sometimes go back off and I can get back to playing with DD.
It's not ideal for DD because I play with a baby strapped to me the whole time which can sometimes be a bit restricting. But I figure it's better then me disappearing to settle/re-settle him all the time and I like having him close and carrying him everywhere.
I think it's perfectly reasonable to want your kids to get enough sleep AND to want to spend quality time with your older children. My DS is ratty if he doesn't sleep and my DD is ratty if I don't get time to play with her. So I try to meet both their needs.
I hope you find some way of cracking the 45-minute nap thing OP. I think that some m'netters have had success with the wake-to-sleep thing (do a search if you don't know what I'm on about!).
If not, then maybe try a sling?