Your grandsons are not eligible for any kind of IRA until they have earned income. UGMA & IRA types cannot be the same account. UGMA accounts belong to your grandsons the minute they turn 18, & they may spend it in a way you'd not want. It also could act as a disincentive if large enough; I've seen this happen.
Consider a simple revocable trust to hold your grandsons' investments -- you get to specify additional conditions in the trust, and administer it as trustee. We set up such trusts for our son & nephews. Our trusts specified discretionary payments for education or other support, with a portion payable upon university or trade school graduation; in the event they didn't pursue higher education or had funds remaining, the balance paid out at age 30.
We invested the trusts in indexed funds like VTSAX; they have the advantages of low fees, excellent diversification, tax friendly, & decent returns without much monitoring. Individual stocks are much riskier. The trusts require their own return, but the tax has been minimal or zero due to fund appreciation; when sold, it's a long term capital gain. The beneficiaries are not taxed on the proceeds.