I think the problem is likely to be the kids as much as anything.I once had to look after a school garden so I know they can be quite active in destroying things ! I think you will need to find something pretty large otherwise your kids will probably just tread on it, for that reason I wouldnt recommend small herbs. Also something with a bit of a defence against kids but not nasty enough to really hurt them. What about some of the many kinds of Berberis ? I quite like darwinii as it has pretty purple foliage and yellow flowers in spring.It has a few spines but not the nasty sort that get caught in your skin.Your kids should soon learn to avoid it however. It's drought resisitant and should be hardy enough to survive the Danish winter. Another possibility is Rosa rugosa; that survived the kids in England pretty well.

It depends on what the children do on the heap: making highways, riding bicycles, inviting the neighbourhood in for serious events are not exactly plant friendly!!!

Thyme probably wouldn't take the wear. Nor would chamomile.

If you live where there is a farmers' supply place, ask for a grass mix that will withstand grazing pressure. I'd say ryegrass and various clover species but it may not suit your climate. Set your mower or 'strimmer' quite high to get the young grasses to spread and to ensure it doesn't get trampled to bald patches.

You could also think about rosette forming plants: Hieracium, Verbascum, Dandelion, daisies - ordinary Bellis perenne, which are all tough enough to be pioneer species on the clay, and will help to develop some structure because of their deep roots.