Peace be to you,
Tell the believing men to lower their gaze and to guard their modesty. That is purer for them. Verily, Allah is acquainted with all that they do. And tell the believing women to lower their gaze and to guard their modesty, and not to display their adornments, except that which ordinarily appears thereof, and to draw their head-veils over their necks and bosoms, and not to reveal their adornments except to their own husbands, or their fathers, or their husband’s fathers, or their sons, or their husband’s sons, or their brothers, or their brother’s sons, or their sister’s sons, or their women, or their slaves, or male attendants free of physical desires, or small children who have no sense of women’s nakedness. Let them not stamp their feet so as to reveal what they conceal of their hidden ornaments. And turn unto Allah altogether, O you Believers; in order that you may attain success." An Nur (24:30-31)
Thats the verse for hijjab.... Since your christian, obviously you don't have to wear hijaab.... (side note: although I did read somewhere about a hijaab command for the christian women)
I think the mother of that kid may not have a problem with you... but a problem with her son. She may not trust him to be alone with other women.
She may be trying to minimize the female contact for him to avoid any sort of fitnah.
The answer to your question however is both.. its cultural and islamic. Quran explicitly states whom a muslimah can reveal herself to and who she cannot.
As far as culture goes... you can never get away from it... but some people take it lightly and some people take it seriously depending on what you have been acustomed to...
I hope everything works out...
peace and blessings,
Hamid