I have rechecked some episodes of season 1 and found Erin as assumed.
Pic 1 is from s1e8 in wet pigtails. Pics 2 + 3 from s1e5 at the beginning. Pic 3 also showing young, redhaired Elizabeth.
Erin’s pigtails are long but not long enough for tagging as such. I think she rarely wore pigtails, I think mainly or only in the earlier seasons. I prefer Elizabeth here in later seasons (5-6) very often in pigtails, sometimes with glasses.
Main entry including teen girl Mary Ellen, rating 10 at