I´d like to know if the average salary for a PostDoc and a PhD student is sufficient for a middle income expectations in Denmark. What I mean for middle income is for the two of us (1 PostDoc and 1 PhD) to rent a 1 bedroom flat, spend in public transport, go to the supermarket without worries and travel on vacations once a year.
Cost of living is high, particularly in the Copenhagen area: rent, food, transportation, etc. But already with a PhD salary you will get by no problem and you will have plenty of extra money at the end of the month. Two salaries will allow you to rent a nice small apartment, so no problems. People with kids do PhDs in Denmark, so that should give you an idea, if you have no kids it's plenty enough.
This answer is about the København/Copenhagen/Kööpenhamina region.
As a foreign postdoc it is possible to get a flat tax rate, at the price of losing all deductions. This might or might not be worth it financially, but certainly makes planning easier.
Renting an apartment is terribly expensive. First, it is difficult to find one that has a reasonable price. Being a foreigner does not help. We (two adults and a child not of school age yet) had to first live in a far too big and expensive apartment before finding a cheaper one. A couple just moving in are renting an apartment with three rooms. Cheap apartments do not exist and finding one of reasonable price is not trivial. Both of the examples here were of Nordic couples/families, and the latter could read Danish fluently and write it intelligibly.
You should have savings. It is typical to pay three months' rent when moving in; the first and the last months of the contract and one more just in case.
It is typical that the walls of an apartment and newly painted and you are supposed to paint them again when leaving. This, too, is expensive. Try to avoid this at all costs when making the contract; it might be possible with a short one, but might be difficult with a longer contract. Be very diligent when making the move-in report and take detailed photographs of everything, just in case.
My family lived with a single postdoc salary. After getting past the initial shock of settling in, we did not have problems, visited museums and places for children and used the public transport.
We travelled back to Finland for Christmas and summer vacations.
Some caveats: We did not eat out (though I did eat at the DTU cafeteria), but also did not save from ordinary food expenses, eating plenty of nuts, for example. We do not drink alcohol. I walked and biked to work, so we did not use the public transportation daily. We did pay for the daycare, which is an expense lacking from you. We tend to buy things used and I tend to not buy many non-food things.
So, given a reasonably frugal lifestyle, we did not have problems living with a single postdoc salary. I did have enough savings to not worry about it anyways. We were there for little less than a year; a longer stay would have made things easier.
PhD salary is quite comparable to postdoc salary.
It is unusual to eat frequently at a restaurant in Denmark (and Finland and Norway, so extrapolating to all the Nordic countries might be fine, too). It is something done on special occasions, not every day or every week, for most families. It is also expensive, as the staff has reasonable wages and there are taxes.
Eating out seems to be bizarrely common in some other parts of the world.