Yes, those covered on by their parents' insurance plans can participate in the QSEHRA, as long as that plan is qualified. Remember: employees must be covered by insurance that qualifies as Minimum Essential Coverage (MEC) to receive QSEHRA reimbursements.
However, employees on their parents' insurance plans cannot claim reimbursement for their insurance premiums.
This means that, if you have chosen to allow for medical expense reimbursement as part of your plan, then your employees on their parents' compliant insurance plan can use their entire allowance to claim reimbursement for medical expenses. On the other hand, if your QSEHRA is set for premium-only reimbursement, these employees will not be able to claim any reimbursements.