YABU. My parents help with private school fees and I am incredibly grateful, however there is no "debt of obligation" involved, nor is there any controlling behaviour or anything behind it other than love.
It is happening because both my parents were pretty much ditched by their respective families with no support, and it was hard getting set up in life, and even then there was still carping and sniping "tis far from X you were reared..." going on. I am an only child, and it gives my father a lot of pleasure to be able to contribute to his grandchildren's education, and while the state schools around here are fine, the private schools are outstanding and our children are having a great time. Going from (the lovely, but overcrowded) state to private has been very positive for us.
Do some homework before you accept though, because as said upthread, the extras can add up, and secondary school costs more than primary, so £20k - £30k a year for 2 kids in primary can easily become £30k - £38k or even £40k a year and that's a BIG hike + school trips, 85 different sports kits etc. (We're in the SE - I know there are cheaper schools in other parts of the country - I'm just citing the worst case scenario for fees).
Also, I assume the grandparents have thought this through, but if they drop dead when your oldest is in year 9, will there be provision? Do they understand the increased cost of secondary school fees? "Oh yes, we'll pay for private..." could turn into "OMG - that's insane. So you will have to pay half." and then it all gets weird and disagreeable.
Of the private schools around here, some are v wealthy & showy, others are far leaner with parents who are making huge sacrifices to send their children, and it's worth finding out which mindset suits your family better.
YABVVVVVU to refuse such a generous gift out of hand because it is grossly unfair to deny your children such a potentially fantastic opportunity that may well impact them positively for their whole lives, just because your family was poor when you were small, but you YWNBU to pin down exactly what this means, the escalation of fees, who pays for extras.
As for being in hock, you're not buying a tv from the local moneylender, it's a very generous gift from a loving grandparent. Don't let people on this thread make it seem tawdry or controlling.