while she has learned the past tense, she generally only knows the regular form.

The normal pattern among children learning language for the first time is to learn words in isolation and only later pick up the rules. Perhaps, because you may not have as much exposure to your granddaughter as you did to your children, you missed the stage where she was using irregular (strong) forms of verbs, only noticing when she learned the rule for weak verbs and began applying it to all verbs. Some children even use the strong form of a verb but apply the weak past tense marker anyway, e.g., "I sawed a man." She doesn't mean she tried a gender bender version of the magician's trick.

The important thing for you to remember, this isn't a step backward; this is a step forward.