I want to apply for a parent´s uk visa, I am the mother of my child who is a British citizen living in UK

[FabiZul]

Hi all, hope someone can help me here for I have so many questions and my English is not that great. My son is 16 years old and a British Citizen and he has been staying and studying in the UK for a year now, he is currently leaving with his grandmother, this is the mother of my ex-partner, we never married and we are separated a year ago. I am currently living in Mexico and I ma planning on using cash savings which covers the amount specified in the immigration law to proof my financial requirements, the savings are a gift made by my son´s grandmother and she is the one who will provide accommodation for me and my 9-year-old who is a British citizen too, we will all be staying at the grandmothers house, would this be a problem when I apply for the visa? Also, my kids father gives me some financial support to cover expenses for my children as we agreed without going to court, is this a problem too as seen as it looks like I don´t have a sole parental responsibility? I am very confused, wish someone can give any advice or direction on these matters. Thank you!