If the children are exposed to drugs in their father's home, you can immediately go into court with the proper evidence and ask for either supervised visits or suspended visits. It seems like there is a lot going on, but in order to change any existing order, you have to have a change of circmstances that effects custody or visitation. If there is physical abuse in front of the children at dad's house, that, too would qualify as an emergency.
Prior to bringing this matter to court, you might discuss with a lawyer contacting Child Protective Services to make a surprise visit to see just what is going on in this home. Their verification of the circumstances there would mean a lot in any later custody proceedings. The drugs, dropping them off too early at school (thus leaving them unsalfe and unprotected) are of great concern. You don't say how old the older children are that are being obliged to watch the younger children, but if they are too young for this responsibility, that is child neglect right there. You also don't define "mistreats" the children. Do you mean physical abuse? Do you have photos of bruises or cuts?
I would think that under the circumstances, you would be well advised to invest in paying a family lawyer for one hour of his or her time to lay out the story and get advice on what your best strategy and course of action is. You should also consider the possibility that as the childrens' mother you could be held responsible for NOT taking action to put a stop to this abuse and neglect. So get moving now.
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