ASFKAP
I don't know what country you are in but in the UK it is in fact illegal to make any copy of a DVD, for backup purposes or otherwise. Why? Because you have to break the CSS to copy a protected movie and that is against the law.
AFAIK!
However I suspect that making a copy of a dvd that the kids watch to protect it will not generally cause any problems.
http://www.opsi.gov.uk/si/si2003/20032498.htm#24 refers.