Hi, is there a way i can flatten my grass drainage hole so kids can play to avoid getting injured.
Welcome to the Bunnings Workshop community and many thanks for your question.
I'm sure our resident D.I.Y. expert @MitchellMc will be happy to share his thoughts when he is back on the site tomorrow. Let me tag some other helpful members who also might have some suggestions for you: @Prawns, @TedBear, @Chaks_DIY, @redracer01.
We're really pleased to have you join us and look forward to reading about all your other projects and plans for around the house and garden. We have helpful and creative members sharing advice and inspiration on the site every day, so we're sure you will get loads of value from joining us. Feel free to post anytime.
Assuming that you want the area to remain flat for playing on,... you will need take control of the height of the grates' tops.
Perhaps you could do that by digging around the existing grates and installing a shallow storm-water pit (with its own grate) to surround each existing grate, with the top height set so the new grates are aligned with the top of the lawn.
https://www.bunnings.com.au/everhard-325mm-series-300-shallow-stormwater-pit_p4771398 (Will need to buy a suitable grate for it separately. That's the expensive bit.* )
If that is a possibility, (I would do a test dig first to see if there are any obstructions in the way) I would dig down until the drain box sit just proud of the lawn (~25mm above) and knock it down the last bit to the correct height with a rubber mallet, to ensure it sits firmly in the ground without sinking. Then back fill it. The pit can contain dirt, since it is just a holder for the bigger grate that your kids can run on.
* Without knowing the existing grate size, perhaps this product would fit over your existing drains and raise them to the correct height (a lot cheaper and simpler, if it does suit...
It's great to see you've already received a helpful reply from the knowledgeable @TedBear, and that approach sounds like it would be successful. You'll need to raise the existing pit to be level with your lawn and backfill around it. Bear in mind that raising the pit's level reduces the amount of water it removes from your yard. Whenever I see multiple pits positioned this low, especially in a backyard with a retaining wall along one side, I question the reasoning behind it. My first thought is that the installer expected a vast amount of water to pool in your yard due to being below the retaining wall. If you raise the pits, level with the grass and the house, there's a risk involved in extreme weather. However, this could have also occurred due to the pits being installed before the topsoil was added, and as such, they are now below the height of the soil.
Another option might be to leave the pits where they are and create a platform over them. You could place a Good Times 1113 x 555mm Grey Stone Ekodeck+ Pre-Fab Panel over the top and level the soil up around it.
Let me mention @Adam_W to see if he has any thoughts.
Building upon what @MitchellMc suggests as being the reason for the drains, perhaps you could install a channel drain across the house entrance area to prevent water flooding in. Run that drain's outlet to the closest existing soak well that those existing drains feed in to. Then you may not need those drain holes in the lawn at all.
Might be possible to add a "coupler" to the old PVC pipework? Cut the coupler to suit the height you want so the old grate is flush with the grass? (assuming 100mm pipe https://www.bunnings.com.au/holman-industries-100mm-dwv-pvc-straight-coupling_p4750199 ) normally you would add extra pipe to the coupler - but you dont require much extra height at all so the coupler itself would maybe do the job to increase height
I installed somthing similar at my house but in my case it was simply a exit for water from my house gutters - I installed my grate flush with the lawn probably a few years ago - but over the years mines dropped a couple inches of height as well and now also looks like your current ones (I'll have to do somthing to mine soon as well - might be a ongoing issue)
Edit - Just thinking when i was installing mine - I had a little bit of leeway with the height of the grate just in the fittings I had used. Perhaps after digging around your existing grates - you might have enough "leeway" with your current connections to bring the grate back to flush again? I cant remember if I glued my final connections or not to be honest (so maybe mines as simple as a bit of a wiggle of the grate to get it back flush again lol)