Praying for you all! This is so hard to navigate. As the daughter of a Buddhist-who-became-Christian, I definitely relate and would strongly recommend not having your son touch the offering. As much as is possible, I would lovingly but firmly explain why your family is Christian and why the invitation is against your religious beliefs- just like you would never try to force your faith upon them as Hindus. Please feel free to message me if you want to discuss further!
First I would pray a hedge of protection over your son and family, I would pray for your mother in law and your husband family a hedge of protection over them and mercy upon them…I would also start praying that his whole family will come to Christ…definitely show your appreciation and affection for your MIL prayers but don’t feel like you have to oblige with your son because you have set a boundary that your son will not be used to please your MIL and husband’s family…talk to your husband about this in depth…as Christians we know that false gods are really ancient demons…I would do some spiritual work and renounce all witchcraft and all Hindu gods and every sin in your bloodline and pray over your son especially about his future…the enemy is after our children because they are the ones that is going to expand The Kingdom of God…pray and say in Jesus Name and by The Blood of Jesus at the end of every declaration and prayer
Please do not let your son go to the temple because that would mean in the spirit realm your son would be an offering to the demon (Hindu god)…above all else ask God and seek His answers
I wouldn’t do it unfortunately, even to keep the peace within your family. Remember Christ came to sow division between brother and sister, mother and son, etc etc. If you are walking in the light of Christ, unfortunately that may mean you ruffle feathers when sticking to your beliefs. Obviously be kind in declining, explain how important it is to you not to worship or even dignify other gods. Hindus worship many gods with no issues, so the monotheistic view is foreign (as I’m sure you know). Your MIL may not understand, but the most you can hope for is that she’ll respect your decision ❤️ You could also ask Jesus’ mother Mary to pray to God for your family and your son. God said there will be amnesty between the Serpent and the Woman, and in revelation the Serpent’s head is crushed by she who brought the Son of God into the world. If anyone can ask on your behalf for Gods protection against the devil, she can.
Unfortunately you cannot come into agreement with the worship of other "gods." Jesus wouldn't make such a gesture, it would deny His own self and Father/God.
Stay true to your relationship to Jesus regardless if you feel guilty
I would absolutely not partake in this. The Old Testament shows us how God feels about sacrifices to other gods. Not only it’s adultery and idolatry in God’s eyes but what your mother in law is trying to do (even tho she’s not aware) is making a COVENANT to a demon in your son’s behalf. This is tough cause you don’t want to hurt her feelings or offend her however this is your son’s life and you don’t want a door opened over his life to a pagan god (who in reality is a demon disguised). Pray about it and ask the Lord to give you the best way to deal with this but as your sister in Christ I’m encouraging you to stand firm for YOUR God. & maybe use this as an opportunity to preach the goals to your in laws.