What Age By Law Can Child Decide To Go With Other Parent In Missouri

(Missouri)

My step children do not want to have to go to their Mom's every time they are suppose to due to her lies, sometimes physical abuse, verbal abuse, and constant bad mouthing about their Father and I. Is there a law in Missouri that at some age the kids have the right to say I do not want to go and they do not have to be made to go? We thought it was 14. We thought that we had to make them go until they were 14 unless the Mom said ok and then after 14 the kids could just say no I don't want to go and they didn't have to. We want to help the kids and not have to put them in the situations their Mom does, but we also do not want the cops at the door causing the kids more stress and making them go with their Mom after she is mad they don't want to go. Please tell me what we can do to help them and if there is a law that protects their rights.