My advice for training young goalies off the ice, is "Don’t train young goalies off the ice!"
I don’t think young goalies should be encouraged to spend time in the gym or specifically training off-ice. Hockey should be fun for kids, and pushing them into workouts that they don’t want to do is not fun for them. A great quote I heard years ago was “Your kid won’t make the NHL because of what you do for them. They will make the NHL in spite of what you do for them.” That means you can’t force your kid to the NHL no matter how much you want them to get there - if they get there, it will be because of their genetics, luck, talent, and how much THEY want it.
What young goalies should do is play as many sports as possible to develop their athleticism. A 9 year old goalie shouldn’t be going to goalie school year-round. They should spend the summer playing baseball, soccer, tennis, or just running around with their friends. Get them to be active and outside, and get them away from their video games and phones. If you want to develop a great goalie, develop a healthy person. That means sunshine, exercise, fresh air, and movement.
And if they love other sports, that's great! Playing baseball develops your glove hand. Tennis builds endurance, agility, hand-eye coordination, and power. Soccer builds speed and fantastic cardio.
But, if your goalie really wants to train off-ice, then that is absolutely fine. Here is what they can do.
Sprints
Jumps
Medicine ball throws
Distance running
Squat Jumps
Squat holds
Jump rope
Hip mobility and stretching
Core work like situps and planks
They can even lift weights - but they should have someone qualified there to teach them and watch to make sure they don’t get hurt. The reality is just that most young kids don’t want to lift weights.
My advice - have the young goalie play lots of sports, and around 13-14 they can start to focus on building strength in the weight room, increasing explosiveness and power, and building up their conditioning. But the sports will give them a great foundation - for all athletic qualities and for life.