We have 3 kids and did what money we could at the time. We're a bit more financially stable now and our middle kid is a bit behind, so I've projected what I think college should cost and rate of inflation and will give our middle son a $1k/year raise for the next 7 years I think, then all should be "fair", however things always change, markets go up and down, and there is no way to make things exactly right. I am in VA and when I started doing research for our state, you can get state refund credit of up to $4k/year per account, so we opened up 3 accounts (1 for each kid). We couldn't open an account until our son's were born, as we needed their SSN I believe. We do about $3k/year for each kid ($4k/year for the middle since he is behind) but that gives us a good state refund (which I calculated at like 4% tax advantage - I think I got the math right). So with no gains in the market each year, just by putting it in it's 4% growth. The 3 separate accounts let us get close to maxing out and we're keeping things as fair as possible, but based on your post I don't think you'll have to worry much about how fair it is at their college freshman year since it sounds like you guys will be in a very good spot.
My boys are 9, 7, 4 and I have done some research into how to try and get the best financial aid/grants/scholarships, and retirement accounts and home value (on primary house) don't count against you, but savings and post-tax accounts do, so my plan was to continue maxing out all retirement, try to have house paid off completely before 1st fills out FASFA forms and hope that we can get some grants/scholarships, maybe that'll be a good year for my DW to retire to show less money coming into the house.
Longer answer to your question, but even if you aren't going to max out, I would open a separate account for each kid, but you'll want to read into your state's law's, as for VA having a separate account for each kid makes sense. I think you can even open up a separate account for each spouse for each kid, I can't remember. But at 3-4k/year per kid that's about all we can handle and that should come close to providing a full scholarship for each kid assuming 8% growth and state school at around $25k/year at 3% increases/year. I'm trying to keep things fair, but in all likelihood across a 5 year span between kids, the market will go up or down and honestly we'll probably make up the difference.
I have thought more now though about whether it would be better to not fund so much and let them take out student loans for some of the amount in the event that they got a job after college with an employer that paid off some of their student loans or something? You never know what the market will be like in 20 years and what companies will be offering, if all of your kids will go to school, etc. I figure if one of my kids didn't go to school or got a scholarship then I'd happily pay the fines for taking the money out, or shift the money to one of the siblings or for graduate school, or even seeing about transferring those accounts onto their kids if that's possible. Lots of ways things can go for your kids and planning out different scenario's can only help you be more financially prepared.