You may not keep a calendar, but think about your weekly schedule. Think about all the appointments and work and meetings and school and kids events and clubs and ...
Now, is anywhere in your schedule, "Play w/ kids" or "cook dinner w/ kids"? Aside from shlepping to and from various practices or doctors appointments, do you have any time on your schedule that is actually set aside for your kids?
For a long time, I've worked this way with my kids. When an open block of time appeared in my schedule, I'd mentally assign it to spending time with the kids, but I wouldn't actually write it down. Then, inevitably, something else would pop up and I'd fill it in to that open time and time with the kids would vanish despite my best intentions.
Once we schedule something, we plan other things around it. The other things competing for that time in our lives tend to take a lower priority to the thing that we've already planned. When a conflict arises, it's on us to choose which activity is more important and which gets pushed off to a later date.
Actually schedule time to be with your kids doing something fun for them. Once a week is great and can still be realistically accomplished. Put it on the schedule and then defend that time aggressively. Don't let other things crowd in on it and take that time from your kids.