Sleepers Required For School Playground Plant Beds

Stockbridge Parent Council
Description
Stockbridge Parent Council and Stockbridge Primary School are looking to improve and maximise the potenial of the limited green space at the school, for the benefit of the pupils and community. They are embarking on a project to plant native hedges and replace the current sleepers which retain the current borders, before the end of the school year.
About the Parent Council and school:
The Parent Council is a group of parents and carers who represent all Stockbridge Primary School families to the school and wider community. Stockbridge Primary School is a non-denominational local authority primary school in Inverleith ward, Edinburgh. Our current roll is 258 children from Nursery-P7.
About what we want to do:
As with many city schools, Stockbridge Primary School has never had a lot of outside space. In 2017 this was reduced further when two new classrooms were built to accommodate rising school rolls. In order to build these extra classrooms, the only green area of the playground was removed, including a number of mature trees.
The playground had been well used by local families as it was a pleasant, green, safe space. It is an open playground that can be used by anyone in the community, outside of school hours. Repairing the bed edging will allow us to plant a mixed native species hedge that would bring back some greenery to the playground that has been lost, provide environmental benefits and create outdoor learning activities for the children.

Previously the bed has been edged in board which is now falling apart and soil is being lost. We cannot proceed with any planting until this is repaired and the soil is protected. As this is a school playground, we would like to replace the board with a more robust edging, like sleepers, that would allow the children to sit/balance on them without risk of dislodging them. These do not have to be new and we are open to advice on what would be best to use.

Requirements
We need to replace 30-40m of edging board along the front plant bed of our school playground. We would like to do this with sleepers or similar that would stand up to children climbing/sitting on them, as well as do the job of holding the soil in place. This would allow us to plant a hedge of mixed native species that would give the children outdoor learning opportunities, increase local biodiversity and improve the playground and immediate environment.
If you are able to help with materials or installation we would love to hear from you. We would like to complete this as soon as possible, so that we can plant the hedge with the children before the end of the school year (June 2022).