The approval process on parent object should check number of child records and greatest of discount percentage on them, I could create roll up summary fields but our org has exhausted limits. The criteria is to send opportunity for approval when more than one reservation is of specific type and greatest of discount % is More than 50%. Opportunity is parent , reservation is child (type and discount% are its fields) Any help is appreciated.
I would try to have a shot at Process Builder (Setup > Create > Workflows & Approvals > Process Builder.
You could create a process flow on Opportunity Product based on your condition "discount > 50%". Then you can actually update a field in your parent object, in this case opportunity. This field could be a simple checkbox such as "Approval Required".
Then in your approval process on opportunity, the criteria will just check if the new checkbox is true.
Potentially you could then have another process flow on opportunity when this new flag/checkbox is set to true, add a submit for approval action which is something you can do with process builder and not with workflow.
Process builder is quite powerful compare to simple workflows