If you would like to enable creation of both of these records at the same time, you will need to create a Flow or a VisualForce wizard. I have more experience with the latter, but believe the former is more configurable and overall lower effort.
EDIT As noted by SFDCFox, you will need to have a clause that checks NOT(ISNEW()), because the requirements you list do not allow for Account creation in the first place.
You can accomplish this purely via config! Simply create a rollup that sums the child records and then a validation that makes sure that rollup is > 0. This does not work for lookups but is achievable because the relationship is Master-Detail.
This solution can cause some updating issues if you have Apex logic around the relationship, in which case an Apex Trigger can certainly address this need instead. Please post a code sample if you would like Apex help.