I want to do joint custody to be fair, but

my stbxh and i have 3 little girls, ages 7, 6, and 5 (youngest girl is developmentally disabled) as a sahm i am the primary caregiver to them, i prepare their meals, do the cleaning and bath time for them, help with homework and projects, schedule play-dates, and outtings, and dr. appointments, and whatever else i need to. at this point, the only thing my stbxh has to do is drive them to school in the morning. (i pick them up in the afternoons) lately stbxh has been claiming that i refuse to let him spend ‘quality’ time with the girls without me, and i’m attempting to turn them against him. it’s true the girl spend the vast majority of the time with me, but that’s also because i’m the one who actually does stuff that’s appropriate for children with them. the last few times my stbxh watched them i got very upset with his choice of activities. back in january he watched the girls while i went to a baby-shower, his idea of watching them was dropping them off with his little sister while she was trying to study for some tests and hanging out with his buddies. before that, the last time he watched them, he thought it was appropriate to take them to a junkyard and scrapyard while he rummaged through old car parts. i’ll admit i was rather upset about that because of old, rusted metal, and the fact having 3 young children in a place like that simply strikes me as being unsafe. they could’ve easily gotten injured. their’ shots are all up to date, but still… anyways, i want to be able to work out a custody agreement that is fair, because i feel that my stbxh needs to spend more time with the girls, but I seriously question his choice of activities with them. anyone have any ideas or advice how to handle this one and be fair to everyone? thanks!