I did separate for my kids and have already given them their books. More work for me, so much easier for them. And I have a set of books where everything goes so I'm making some duplicate pages now.
Making a book for granddaughter and it is pictures I've scrapped which they sent me. However, if we are in them or if it is really special, it goes in my chronological book too.
I like paper and scissors layouts, but that is me. I don't strive to be a great scrapbook artist either. That just isn't important to me. I want them to look nice and be something my family can be proud of.
I also want to get my things in a book chronologically and seldom take the time to b really creative because I scrap so much! We travel a lot so I alwyas have a stack of things to scrap to keep current.
But I don't consider it a waste to buy paper and adhesives, that is what I love about the hobby and I love the dimension. Its nice when my pages are nice and creative, but that isn't my goal.
Yes, put all your memorabilia in the same book.
I spray it first with "Make it Acid Free" so it will hopefully last longer.
I hope you will journal - someday your kids will want to read every word you have written. The pictures just don't always tell the story, there is more to it. Some of my pages are nothing but journaling. I love to read and I love to write. Their scrapbooks are where are their amusing stores are growing up.
Even with my husband I journal our funny stories.