I first read “crosswords” and got worried because that is a favorite recreation.
if you mean by cusswords, actual cursing, saying “damn you” and meaning it, trying to invoke a curse on someone, that is a grave sin agains both the 1st and 2nd commandment because it assumes a supernatural power belonging only to God and attempts to manipulate it, and of course because of the wish to harm another person.
if you mean profanity and vulgarity, the former is a mortal sin if it denigrates the human body and the natural function and beauty of the body as designed by its Creator. (the words we do not type on a family forum that will be ** out by the eidtor)
Vulgarity can be mortal or venial depending on factors like ignorance, intent, scandal etc. To the extent it is a verbal attack on another person, it can be gravely sinful. Jesus says anyone who curses his brother or even calls him “blockhead” commits murder.
We forbid our children to speak this way because we want them to love others, to obey the commandments, and to grow up courteous, polite, literate and respectful members of society. We do not want them to display ignorance and crassness, and even more, we do not want them to sin.
If one needs to use ordinary scientific, medical terminology in ordinary conversation use the proper word in the context intended, and teach children the proper words, not popular vulgar slang, and teach them to respect the words as they would the bodies being described.
If you mean using God’s name in vain, again sins against 1st and 2nd commandments, gravely sinful to the extent it is willfull, deliberate and there is intent to blaspheme. We teach children to obey these commandments because we teach them to know, love and serve God with all their hearts, minds and wills, and this language is not compatible with that conformity.