Here is my honest opinion, borne out of experience. As much as I love coins and bullion, I would not advise you to "stack" silver or other precious metals for a number of reasons. Silver and gold are not great long term investments, unless you happen to time things right. They don't go perpetually up, they wander with the economy, peak once a generation, and drift back down. They don't pay a dividend, you can't eat them, and there is plenty in the Earth's crust and in private and government safes to meet demand and suppress rising prices. (This is all similar to the price of oil and gasoline.) They are also surprisingly illiquid investments when you go to sell; unless you are Englehard or a refinery trading 100,000 ounces at a time--or dealing with someone like me--you'll be hard pressed to get more than 70-80% of the melt value when you go to sell.
I think silver and gold are good investments as a small portion of a portfolio, 5-15% maximum, as a hedge against inflation or a stock decline. To be honest with you, I trade in the stuff but don't hoard it myself. That should tell you a lot. For my kids, if I had the money to give them (this isn't the dream job everyone thinks it is, maybe I'll leave something for those future grandkids) I would buy no-load Vanguard and Fidelity index mutual funds. Or simply gift them the money annually and keep tabs on how they handle it. And if you do want to invest in these vehicles, you are best doing it on the commodities exchange or in a mining stock; mining companies are leveraged with the metal prices, pay a dividend, and are easy to buy and sell on an exchange for reasonable fees. Your grandchildren will, if they are like most families I work with, not going to be qualified or interested or have the time to do what it takes to recoup the sparkling investment you imagine leaving to them. And frankly, if they are my age or younger (My folks tell me I was at the 1964 World's Fair in a stroller), life is expensive and they could use the money now; if they can spend it now and bank their paychecks, or you can help them set up IRAs, you'll give them something that will truly grow over time.
A good strategy for the above is to "dollar cost average" by buying a set amount every month or two rather than all at once, to protect yourself from unanticipated market swings.
If you firmly believe you wish to invest in silver, gold, or coins on their behalf, I would love to guide you. You should acquire these metals in the best .999 form you can at the best price. I don't want to see you paying more than a few percent above spot. I always have buyers and sellers ready to deal, and I work on a narrow spread. In fact if you place a buy order with me (no deposit required) I can probably get you your Eagles and Kruguerrands BELOW melt value within 3-6 months. As you purchase you should also leave clear written instructions as to what you paid and how it should all be sold. I can help you set up this collection responsibly, or you can create a trust that I can manage or advise. You can also invest in problem-free, certified, high grade coins. If I could go back in time, I would take every $1,000 I spent as a young man on 50 coins and combine it to buy one coin a year at $5,000. The best, the rarest, the problem-free specimens, like real estate, will always rise in value because they are always in demand.
Please call me at 914-649-33147, should you wish to explore these options further. You have five grandchildren who are very lucky to have a grandfather who cares for them as much as you do.