DCL does not state that children 4 and under must be vaccinated (obviously, they can't be as of yet), or that they will be required to be vaccinated once such vaccinations become available. The vaccination requirement as of January 13th will be for ages 5 and up.
The website also states that "all vaccination eligible guests must be vaccinated", but that is a current requirement based on current vaccine eligibility, and is not necessarily applicable when new vaccination eligibility occurs. Legally, it would be a stretch for DCL to argue "the guest should have been able to figure out that's what we meant", and given their history of offering refunds every time a new explicit requirement is added, it is extremely unlikely they would not give guests an out who don't want to vaccinate their infants & toddlers if and when such vaccination becomes available.