You're paying a guaranteed 7% interest rate after taxes to *maybe* make 10% before-tax on VTI, and who knows what will happen with FB and TWTR. Even though payments are not due, the 7% interest is still accruing (well, it may not be now because of 0% student loan interest with coronavirus depending on the type of loan, but it probably will start accruing again sometime soon). After paying federal and state taxes, you're probably not even winning the arbitrage game you think you are.
If I were in your shoes, I would cash out as much FB and TWTR as was necessary immediately to pay off all existing college loans and be thankful that I found something I could invest in that was giving me a 7% after tax risk free rate of return. I would not take out any more student loans going forward as long as I had funds left to pay for college. I'd also strongly consider diversifying away from any remaining FB/TWTR shares. For my kids college funds, I have the next four years of predicted college money in bonds and anything beyond that in stocks. Since I have enough, I don't need to or want to take the risk of VTI over that short of a time frame.
The other thing to remember is that when those loans were initiated, you and your DS probably lost between 3% and 8% up front in origination fees and loan fees and what not. Each new loan takes that bite as well, whether the loan is unsubsidized or not, I believe. That's a painful hurdle that is mostly invisible because they either roll those costs into the loan or subtract it from the initial proceeds, so it's not as in your face. But mathematically it's still a punch in the gut - think of it as costing between 2 and 8 years of the arbitrage game you think you're winning.