As a few community members have already said or implied, above, as your grandchildren grow older they will inevitably withdraw. My mother in law sounds a lot like you and she did so much for our children as both of us worked, however over the last few years since she turned 16, she increasingly became less interested in visiting as regular as she used to, same with our son, who is now 21; I can relate to this exactly as I went through a similar process when I was a teenager. My grandmother had a stoke around the time I was born and she always said I was the one who pulled her through that.
My father worked away from home, (we'd see him at weekends once a fortnight apart from his annual leave), for most of his working life and my mother was of that generation that survived on 'mothers little helpers (as the Stones so aptly named Valium/diazepam). She had three children between April 1955 to Feb 1959 while my father was serving in the Army when National Service was still compulsory, so she was left on her own and suffered many nervous breakdowns over the first 20 years of my life and was in and out of psychiatric hospitals throughout that time, so I virtually lived at my grandmothers for most of my childhood and early teens; and used to go away on vacation twice a year with them.
Yet I remember not visiting as often as I did from around the age of 17 onward. I would still visit once I left home at the age of 18 (I had a shotgun wedding), but those visits became less and less frequent, simply because life got in the way. That said. I always cherish the time I spent with my grandparents; if it wasn't for them, I would not be in the situation I am in now as they always introduced me to new things and were (IMO) responsible for my successful education (B.Sc, M.Sc and several diplomas) as well as my love of reading (though my English Literature teacher also had a hand in that), plus photography (my grandfather had a darkroom that he would take me to regularly as well as gave me my first decent camera at the age of 8) as well as many other interests that my grandfather introduced to me (e.g. chemistry and music). My grandmother passed away the same day as John Lennon (Dec 1980) and my grandfather passed away while I was training to be a teacher in 1992, but I still think of them both regularly, and when I do I miss them so much.
As others have advised, please do not make a big thing out of this as it will only create negativity, which your grandchildren will remember. They will NEVER forget all of those wonderful cakes you've made and their memory of you as they grow older and you become dust will be forever positive and cherished.