I practice law in California, which is a community property state. You live in Florida, which is a common law state. Common law states are often referred to as "title holding states" by estate planning attorneys, because the mere form in which title is held is frequently determinative of: (i) how assets are divided up and allocated to the spouses in the event of a divorce; and (ii) the amount of freedom each spouse has in making testamentary dispositions (i.e., bequests) of such assets. By comparison, property held by a married resident of a community property state may be community property or quasi-community property notwithstanding that fact that it is held in the name of one spouse alone, and the characterization of spousal property as either community or separate has a profound impact on how it is divided upon divorce and distributed upon death of a spouse.
Moreover, although property received by one spouse via gift or inheritance is deemed to be that spouse's separate property, it is relatively easy to "transmute" (legalese for convert) separate property into community property. Depositing inherited money into a joint spousal bank account that also holds their salary income would transmute the inherited money from separate to community.
The bottom line is that a prenup or postnup is generally more useful (i.e. needed) in a community property state in order to preserve gifted or inherited assets from the claims of the recipient's spouse. But most common law states have "dower and curtesy" laws that provide some protection for surviving, non-title holding spouses. They vary from state to state. But, just as the operation of California's community and separate property laws can be varied by agreement of the spouses, my guess is that the relevant laws in a title holding jurisdiction like Florida can also be altered by agreement of the spouses.
While it is clear to me that your granddaughter and her future husband could enter into an enforceable prenup or postnup, it is not at all clear to me whether you could contractually bind your granddaughter to enter into a prenup or postnup at some future point as a condition precedent to receiving her gift or inheritance. Contracts pertaining to marriage and marriage rights generally receive closer scrutiny, and are occasionally deemed unenforceable on public policy grounds. It sounds like you have multiple grandchildren whom you would like to see enter into a future prenup. I think this is an occasion where you should seek the advice of a Florida attorney. And if a given child resides in some other state, then perhaps an attorney from that state as well. You should seek to minimize the application of any other state's laws by inserting a "Governing Law" clause in the trust agreement specifying that Florida law governs any disputes or legal actions concerning your trust.
You could put all of the money into a single "pot" trust for your grandchildren and structure that trust as a "discretionary" trust that gives the Trustee absolute discretion over whom to distribute to and how much to distribute. And then instruct that Trustee verbally as to your wishes concerning grandchild prenups, so that it would hopefully be taken into account by the Trustee when making later trust distributions, rather than risk putting express verbiage in the Trust agreement that might later be deemed unenforceable. But you had better be careful in selecting a Trustee. You would need to feel very confident that the Trustee will use his or her discretion in a way that carries out your testamentary intent.