Is the top of the bark surface going to be level with the lawn?
If you’re trying to create a soft landing area in case of falls from height then you don’t really want edging boards sticking proud of the surface which is likely to be hard and add new trip hazards in.
You will probably want the bark to be deeper than the depth of the turf too for it to have any impact reducing properties.
British Standards EN1176 and EN1177 are a good reference here – might be able to find info on the RoSPA site too.