They have to learn to do it for themselves; if they do get in a mess it will chivvy them up to be more efficient next time.
I was quite young when I married into the Forces. In those days we got an 'allotment' which we collected at the Post Office so that the wives always got part of their husband's salary when they were away as well as at home. Presumably it doesn't work like that any more, but it was a good idea to ensure that wives were never left without money.
If your DS is in the Army he will be getting a salary so they really need to sort this out themselves; they are young but they are both adults now - and they are parents who are responsible for a little one.
I would like to meet here someone from eastern Europe

