i suggest if you do overhead presses that you do with them dumbbells. the child will not remain still when you lift him and dumbbells will work more on stabilizing muscles than a machine. all upper body exercises are going to help you. you need shoulders and arms to lift but also upper and lower back and abs to stabilize your torso. and then of course legs to steady you. i suggest a full body workout including exercises that mimic the movements you do when you lift him. so if you lift him overhead then you need to do some overhead lifts like the presses and the lateral raises and front raises will be helpful too. i suggest dumbbells for those exercises too. you could do deadlifts which would be great for stabilizing your torso and strengthening all back muscles and squats as well. .
what exercises should i do so that i can more easily lift my grandchildren over my head?