Despite your intentions being good, I think this would be an appalling thing to do to your offspring op. Please don't do it! Give them two lump sums each to use how they see fit to kick in when they are about 30 and 50 years old, and apart from that, leave them to find their own way in life.
I knocked around with two people who had trust funds at university who I am still in touch with now. Out of our group of friends, those of us who had to work, have developed our skills and capabilities, have travelled, lived abroad, established relationships, had children, had the corners knocked off us, met many different people, grown in many different ways and have established careers and had the satisfaction (and yes the stresses) that have come from all of that. We are not the same people we were aged nineteen!
It is striking how little the two who haven't had to work have stayed the same, how they are almost personality-wise in a state of "arrested development". They tell the same jokes and stories all the time because they haven't had, and are not having, many new experiences. They are out of sync with all of us and so live quite isolated lives with not many friends, still emotionally dependent on "home" or the remnants of home now that their parents have died.
They dabbled in businesses that never came to anything when they were young, and had one or two jobs found through their parents friends that they didn't stick to, simply because they didn't have to! They were both perfectly intelligent, one even quite bright, and it seems such a shame that neither have put their good luck or brains to good use, for themselves, or others. Now they are turning sixty or thereabouts, they don't really have much to show for a lifetime of good fortune, other than pleasant living surroundings and a monthly income.
The fact that they were dependent on their families for money also led to a wierd love/hate relationships with their parents and in both cases it led to family problems. And neither have managed to establish long term relationships, first because they were suspicious of people liking them for their money, and latterly because they , I'm sorry to say ...don't know how to put this kindly ...aren't regarded as great catches because they are a bit strange and regarded as "loosers" ...sorry ...that's sounds unkind ...a better way of putting it would be they are not generally respected. No one will criticise them to their face of course because of the money, but that's how everyone feels about them in all honesty. Some of us feel sorry for them. They have had such privilege that's protected them from the swings and arrows of fortune that the rest of us have had to suffer, but sadly , to put it mildly, that hasn't helped their characters in terms of how they relate to others, and that's putting it mildly.