Since the purpose of this joint account is to stash money for the parents, structuring this as a gift to adult children is wrong, in my opinion. One of the joint holders is considered the primary owner who will receive the IRS Form 1099 each year, which must be paid on that person's return. Joint ownership makes inheritance murky.
I believe the better way to handle this is to create a revocable trust on behalf of the parents, with one or both adult children having trustee power to manage the assets in the trust. While I recommend some legal advice to get the wording correct for your state, this kind of trust is not expensive to set up & is easy to manage; I've done a number of these for growing educational funds for minor relatives (I'm not a lawyer). A trust has its own tax id number & must file a Form 1041 each year, but its income is taxed separately & independently from the grantor (parents) or the trustee. The assets in the trust are titled with the name of the trust & the trustee. A trust is not tax free like an IRA but has no funding limits & usually results in lower taxes than your joint account.
Since you already have the money, you can establish & manage the trust without the parents' approval. The trust can be as flexible or specific as you want for disbursements on behalf of the parents. It also can & should specify how the trust should be distributed when both parents die, & this is done outside of probate.