Rav Moshe Stern zt”l (Be’er Moshe 8:36) was asked regarding the validity of three minhagim pertaining to children.
1. Not to let a baby looks at itself in a mirror before its teeth start to come out because the teeth may not come out on time and its speech development may be delayed.
2. Not to walk over children because it may stunt their growth. If one walks over a child he should make sure to walk back over again.
3. Tie a red string on a baby carriage and/or crib to ward off Ayin Hara.
He says they are all valid minhagim that were practiced in his home, and people in general were careful about them. He quotes a Rashba that says that you should not cheapen or be “Mizalzel” in Minhag Nashim Zekeinos (old wives tales) since they are certainly rooted in holy ideas, even if we have forgotten the reason for them.
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1. A mirror might confuse a baby
2. One might fall and injure the baby. (Thus retreating over the baby would be contraindicated.)
3. Tosefta Shabbos 8, 4. It is darchai emori and ossur.
The Rambam, Chovos Halvovos, and others, disagree with the Rashba and advise one to follow the dictates of reason in areas that are not revealed by Hashem.