All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Indeed, you did well by preventing your daughter from wearing that locket which comprised some rites of disbelief; so may Allaah reward you. You are not sinful for just touching this locket but you are not permitted to keep it for its owner as this is aiding in sin. Allaah says (what means): {…and cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allaah; indeed, Allaah is severe in penalty.} [Quran 5:2]
You should endeavor to protect your daughter from being affected by her grandmother; Allaah says (what means): {O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones, over which are [appointed] angels, harsh and severe; they do not disobey Allaah in what He commands them but do what they are commanded.} [Quran 66:6] This is achieved by educating her and disciplining her as stated by the scholars. It is not permissible to act flatteringly with anyone with regard to such a matter.
Your husband should know that the best way to be kind and dutiful to his parents is to invite them to embrace Islam and clarify the truth to them and express disapproval of the acts of disbelief that they do, such as uttering words of disbelief and wearing amulets as you mentioned. He should invite them in a gentle and soft manner and supplicate Allaah to guide them because this may be a cause for their guidance; our hearts are in the hands of Allaah and He turns them how He wills. Allaah says (what means): {And whoever Allaah guides – for him there is no misleader. Is not Allaah Exalted in Might and Owner of Retribution?}[Quran 39:37]
In general, the one who should expresses disapproval of their evil is the one who sees that evil taking Place, pursuant to the Hadeeth on the authority of Abu Sa‘eed Al-Khudri who said that the Prophet, sallallaahu ‘alayhi wa sallam, said: “Whoever among you sees an evil, he should change it with his hand; and if he is unable, then with his tongue; and if he is unable, then with his heart; and that is the weakest of faith.” [Muslim]
If some Muslims do this, then the rest of them are not sinful as this is among the communal obligations. While doing so, one should abide by the conditions of enjoining good and forbidding evil: One should have knowledge, be soft and gentle when doing so, and so forth. For more benefit, please refer to Fataawa 12803 and 13288.
A Muslim should not feel inferior or weak when saying the truth or fear the blame of people.
Finally, your husband has the right to prevent his parents from visiting you if he fears harm from them, as it is an obligation to ward off harm. Ibn ‘Abbaas narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: "There should be neither harm nor reciprocal harm.” [Ahmad] While doing so, your husband should be wise and he should try not to upset his parents.
Allaah Knows best.