They are going to eat the grass but in such a large space it should last longer. Perhaps could fence sections of it off if any needs to recover?
How many piggies do you have?
I think there is more to consider than it just not being harmful. What I have in my mind as a bark I would imagine walking on is going to be quite difficult and rough on their feet and it is still going to degrade.
From a very quick search, children’s play bark is made from softwood and conifer trees - these aren’t things you want your piggies to have access to.
Any wood chip (ie I use snowflake soft chip as bedding) is not going to be the sort of long lasting thing I think you are looking for.
I also assume it’s only going to be for summer access.
Perhaps you are going to add a roof which may make it a bit easier to bed?
I assume you are putting mesh underneath the whole area so nothing can dig in?
(I do have an indoor/outdoor permanent enclosure for my rabbits but the indoor area is laid to foam mats and the outdoor area is paving slabs. Rabbits are different to piggies though so that kind of thing won’t work for the piggies due to the much more nature of their feet.)
Perhaps you could link some of the outdoor enclosures you’ve seen as it may give us a better idea.
We have this old thread but my guess is the bedding is replaced very regularly in this case - it’s just hemp which is going to last a week in good weather and five minutes in rain!
Outdoor Guinea Pig Enclosure