How to know whether the oven's door has loosened and leaking out some energy or is as tight as it was when new?
Unless you have a measured baseline from before your child played with it there's no 100% way to know whether the seal is as good as it was when new - you have nothing to compare it to. If you want to determine if you have a decent seal you need to use your cheek and your ear. 

Oven seals not only hold in heat but sound as well, while your cheek is sensitive to heat and airflow. Put something sizzly in your hot oven and then move your head all around the closed door. If you hear the sizzle (or the fan if you have a convection oven) and feel a blast of heat in an area that's a good indication your seal is not effective there anymore. Use your body weight against the door and see pushing it closed cuts the sound and heat. If pushing it closed works it's more likely weak hinges than the seal. If there's no sound or heat leakage you are still good. 

Keep in mind oven seals do degrade and some oven hinges weaken over time, if you do have a leak it may have nothing to do with your kid.