I assume that you have passports for your youngest daughter and for your granddaughter. Remember that you may not cross borders with minors who aren't accompanied by both of their biological parents, unless you have written, translated, notarized permission from both of them; or a death certificate, if they are no longer living. Some folks get into Mexico without that, then have trouble getting back into the USA. Avoid the hassle and expense by being prepared. Driving is pretty straightforward, but I can give you a suggested route, easy crossing and stopping options if you need them. You will find lots of ads for rentals in Guadalajara online or in the classified sections of the newspapers. Zapopan and Chapalita are good sections in that part of the metropolitan area. Welcome to the forum and to Mexico.