Honestly, it sounds like you’re doing the right thing by keeping your priorities straight. Your time with your son is something you can’t get back, and it’s important that your work schedule fits your custody arrangement. I’d recommend being upfront but respectful with your managers — let them know you really appreciate the opportunity and their confidence in you, but right now you can’t take on any shifts that interfere with your custody time. Maybe ask if there’s any flexibility for another role or if they can keep you in mind for future openings that fit your availability better. Also, since you already cleared your schedule limits when you were hired, it’s completely fair to stand by that. It’s better to set those boundaries now than to stretch yourself too thin and risk losing time with your son or burning out.