Well tbh my first concern is being fair.
By which I mean what you commit to your eldest you should be prepared to match (or rather ring fence now) for the younger.
Any course of action that potentially means that's not possible in the future needs to be carefully considered.
There's nothing like money to cause huge family rifts/resentment.
In your situation I'd strongly recommend you get professional financial advice.
You're talking about significant sums of money here that have the potential to be life changing not just to your children but to you and your DH.
No-one ever wants to think the worst but it very much sounds like all your plans are based on the sun still shining.
Circumstances can change in a heartbeat.
By way of a cautionary tale, my DF's friend gave his DS £250k to get on the property ladder and as a way of avoiding inheritance tax (based on the 7 year rule). Problem was he got married, divorced and property went to the wife. Yes it was home to their grandchildren (so I'm not suggesting it was inappropriate), but their DS will never see a penny of the money his parents gave him which was clearly not the intention at the time of the bequest.
What they could have done was buy 50% of his property so he had a better lifestyle but it would have remained their asset (the downside being IT).
This is why you need to speak with a professional to go through all the ramifications/scenarios in advance.