Whether your own your car or have a loan or a lease on it, to find out if your children are covered by your current insurance policy you will need to read over your policy and check with your insurance company.
If your son and/or daughter live in your household then an insurance company would normally require them to be on your insurance policy to be properly covered. If they live outside of your resident and borrow the car, just as a friend might, then they are likely covered under your insurance policy. Some policies have exclusions so that is why it is always good to check with insurance company before allowing anyone that is not listed on the policy to drive your vehicle.
Since this is a leased car you may also want to read through the terms of your lease to make certain there are not restrictions about who operates your vehicle. If you are uncertain after reading through the leasing documents then check with your vehicle's leasing company to find out if your son and daughter are allowed to drive your leased vehicle.