To counter some sweeping generalizations in the other answers: in some places/fields it is not uncommon and can be unproblematic or even slightly beneficial.
I did my PhD in computer science in Czech Republic. Since the stipend was quite low and we had no big grant, there was a tacit agreement in my group that PhD students actually work ~3 days a week on PhD and use the rest to get a part time job. I've seen similar setups in other CS groups I've interacted with, as well as some in different fields.
On the other hand, when there was grant money, PhD students were usually supposed to work full time on PhD - but then also got an OK-ish (but not great) wage beside the stipend.
I personally benefited from the part time work as it forced me to improve my software engineering skills and those allow me to work more efficiently in my current research area (biostats/bioinformatics). It also gave me the confidence that academia is not my only option in life, lowering stress around funding/job prospects etc. While I had some hard weeks during my PhD around various deadlines, I am quite sure I worked < 40hrs a week on average and still got a good (though not excellent) publication record at the end and finished my PhD on time (4 years as is the local default).
With that said, being expected to work full time on PhD and have another job on top of that sounds terrible. Also note that there is relatively good evidence that people generally do not get more work done when regularly spending more time at work beyond ~40hrs a week - although it may feel like you are achieving more. The exact optimal number of working hours definitely differs across industries and people, but is generally not very high (famously Henry Ford's experiments indicated that 40hrs was optimal for assembly line workers in his plants - presumably this work is less intellectually taxing than PhD). If you work too much, then after an initial boost from working longer, there is always a point when you get tired and end up achieving less per-week than previously, despite spending more time working. So if you have a second job and manage to fulfill your PhD obligations at the same time, it is likely that should you be allowed to spend less time physically at the lab/uni you could achieve very similar results (by virtue of being more focused and making less mistakes/wasting less time) but with much less stress.
Hope you find a good solution for you and your family.