To keep the boys interest when walking around we purchased a tree rubbing sheet for 50p each. The boys spent time finding the correct trees to take rubbing off and it certainly helped keep their interest on our walk. Their prize at the end was a badge.
The grounds were lovely and has over 1200 plant species. There was a small playground but the boys were tired so we didn't get to see this.
If you are going on holiday to Devon, I would also recommend a visit to Arlington Court.