When I saw the title of your question, I was expecting someone much older. Forty-one is not especially old for a PhD candidate, and if you can complete your degree in the standard time you will not be especially old for a PhD graduate. You can find data on US PhD graduates in the Survey of Earned Doctorates (SED), including the median time to completion across different fields. Although the most recent SED Report does not report the average age of PhD graduates, it does report the median completion times, and fortunately programs in computing tend to take less time than in other fields.
It appears from your question that your disappointment stems from an idea that holding a PhD at a reasonably young age makes a person a success, and failure to have achieved this by that time renders you unsuccessful. It also appears that you are allowing comparisons to other younger graduates to get you down, which again suggests an unhealthy preoccupation with questioning your status relative to others. I don't think that's a particularly fruitful way to look at a PhD candidature. Rather than looking at it through the lens of status (or whatever that is) I recommend you look at it in practical terms. Completion of a PhD in a computing field will give you solid research skills in that field, sufficient to do independent research as a scholar in the field. Assuming you can graduate by, say, forty-five, you will have over two decades of professional life (maybe even three) to apply the benefits of that training.
As to the practical problem of supporting a wife and child with only small bursts of money, that is a problem, but it is temporary, and it doesn't sound like such a terrible circumstance to me. The big upside of the circumstance you describe is that it allows quality time between your child and his/her grandparents; you might look back on that later and be glad that your child/parents had that time. In any case, the earnings of PhD graduates in computing fields are quite good, so you should expect to have a reasonable salary bump once you graduate your PhD and move into the profession. At that time you should be able to financially support your wife and child without parental assistance, and you'll have plenty of time to repay everyone involved for the support they're giving now.
What should I do about this?
So long as you're still interested in research, I recommend you forget about all the downsides and focus on making the most of your PhD program and graduating successfully. Put aside concerns about your age, status, money, etc., and be glad that you have a supportive family who can enjoy each other's company while you work to complete your program. This can be a great time in your life if you let yourself enjoy the intellectual challenge you're presently undertaking, enjoy the benefits of the closeness of your extended family, and appreciate the time that is granted to you to do this. Enjoy your research work and your family and put your concerns about money, etc., on hold. Most importantly, ignore issues of "status" and just focus on your own self-development, at whatever pace that is achievable.
It's not clear from your post whether your depression is a response to dwelling on adverse circumstances (and vastly overestimating them) or if it is something more serious. Whatever you do, don't allow these kinds of circumstances to push you into depression, leading to an adverse impact on your work progress and family life. Depression often begets failure, which begets further depression. Contrarily, cheerfulness and stoicism often beget success, which begets further cheerfulness. As a first step, I would suggest you try looking at your circumstances in a more objective manner and examine all the success you have had and all the great things you've achieved (and read some Epictetus to put your problems into perspective) and see if this alleviates your depression.
