5 year old a phone

Hi im wanting to give my 5 year old my old phone he uses it in the house anyway to watch netflix and that ive made sure there isnt anything on the phone that he could communicatewith strangers no social media or even the Internet browser has been taken off. But im wanting him.to take it with him when he is at his dads so he can communicate with me it only has my number on and i can see if he is ok his dad doesnt tell me where he takes the kids even though we agreed on it as i dont like the kids around his side the family as they are controlling and ive had issues with them all throughout the relationship as they overstep my boundaries ( they was the main cause of the relationship ending) i feel like my ex takes the kids to his familys as when the boys comeback they are always naughty and demanding stuff and have tantrums when they dont get what they want as the grandparents would give them anything they wanted and they never told them off for being naughty or if the kids get to much for them and the grandparents get annoyed with my kids they would smack them. Im the one whole has to deal with it all when they come back and it takes me days to get back to normal but they go back to there dads at the week end im concered for there safetyas the grandparents take them places where they are to young to be. My youngest went missing when they was watching them and took them 2 hours to find him. After that i refused them to have or see the kids as there has been over times aswell when they didnt watch my kids properly. I cant stop my kids seeing there dad but i need to know they are safe. He has even bribed the kids not to tell me where they have been but i have a feeling