Q. Is it a sin if we miss Sunday mass if we have a valid reason like dangerous weather conditions or sick child at home?
A. This is a good and practical question that I'm sure many people deal with from time to time. So thank's for asking! I think the answer is very simple. And I think the best way to answer it is to first highlight the reason we are "obliged" to attend Sunday Mass and certain holy days.
First of all, in our day and age, in the culture we live in, there is a tendency to dislike authority and being told what to do. We value our freedom. But sometimes we value it to the point of forgetting the role of legitimate authority in our lives. Parents know that kids, especially teenagers, dislike being told what to do and what the rules are. But this is an act of love and necessary. Similarly, God gave the Church, in the person of the pope in particular, the authority to oblige us to certain days of worship. But we should never forget this is a grace and a good thing! It's a way of knowing, with certainty, what God wants regarding our worship of Him at the Mass. So in exercising this authority the Church has bound us to worship at Mass on Sundays and certain holy days. Again, this is a good thing and frees us from the burden of trying to figure it out all by ourselves.
With that said, and with a proper understanding of the obligation to attend Mass those days then, yes, it's a sin to intentionally miss Mass through our own fault. It's an act of disobedience and also a disregard for the sacredness of the holy Mass. I realize this is hard to swallow for some and our defenses want to immediately go up sometimes when we are told what to do. But I'd want to lovingly challenge anyone feeling that way to realize this really "obligation" really is an act of love from God and His Church.
Now to the heart of your question. What if it's bad weather or you have a sick child or something similar? I don't think there is any black and white answer to this other than to say God is a very reasonable God and would never impose upon us a burden that is unreasonable and too burdensome. So, to be direct, I do think there could be very legitimate reasons someone would miss Mass and it would be the right decision. The obvious example may be the elderly person who cannot drive very well in the midst of a snow storm. Is God going to be angry if this person is too afraid to brave the roads for fear of getting stuck or in an accident? I think not. Again, God is not unreasonable. And the sick child could be another legitimate example.
But be cautioned! Sometimes our tendency is to take advantage of the generosity of God. A good rule of thumb for missing Mass for a reasonable cause would be that you would not go out for any reason whatsoever! So, for example, if you decide to miss Mass because of snow but then go to the movies, or the store or McDonald's then we are the ones being unreasonable and are not being honest with the situation. Some also miss for reasons of inconvenience such as being on vacation. But, again, this is not a reasonable excuse barring impossibility.
The last point I'll make, which could so easily be missed, is that we should never forget that going to Mass is an incredible privilege! The obligation mentality is the minimalist approach. The truth is that we should work toward fostering a deep desire to attend Mass because we have discovered the great spiritual benefits of celebrating it and receiving our Lord in Holy Communion! If we can achieve this mind frame I think we will always know what to do.