Think your mother in law is testing water... So suggested it to you.
My mother in law also like to test water like that. dont't be too bothered by it. As long as you say no, she cannot insist.
If I was in your shoes, even if heavy rain, I will also bring my son home. Sekali your son say, I like nai nai house, I like to stay over at nai nai house.. Then cham. Best is not even have a first time.