Unless there is prior trauma, hiding and sneaking food is almost always an indication that the children are way too restricted and that there are serious issues around food in the home (ie too much restriction). I highly highly recommend that you meet with a therapist who specializes (this is important) in disordered eating. I don’t think there is any other option but to be blunt - your children will develop lifelong eating issues, that could lead to full blown disorders if they haven’t already if you don’t start approaching this in a different way - based on research and with someone trained in this specifically to help you. Your child WILL continue to gain weight if you continue on this path as they will continue to feel restricted and then binge.
Here is a place to start, they specifically also work with people that don’t necessarily have a diagnosed eating disorder so don’t be turned off by that. Regular therapists are NOT trained in eating issues and can actually be harmful sometimes (I speak from both professional and personal experience here) and they are doing virtual sessions right now
https://www.theeatingdisordercenter.com/
I also would be cautious with recommendations here if specific things to do with your kids from folks not trained in this.