Help a total beginner fill a massive trough

[CharityConundrum]

I am involved with a charity which has a massive trough to fill. It's basically a raised bed, but it's about 50cm high and nearly 6m long, so we need to bulk out the bottom and make it suitable for children to dig, plant and play in.

I'm hoping to get some donations from local garden centres etc, but don't even know where to begin with what to ask for. We need something in the bottom to allow it to drain, but what would be best? And how deep does the soil need to be to grow things in it? Any help with what we need to ask for and how much would be much appreciated! Thanks.

I put a layer of polystyrene chips/ broken pieces in the bottom of my planters. It bulks it up, but is light, allows for drainage and is a good recycling use.
I assume there are drainage holes in the bottom. When I was involved with a school garden, we had to get special compost that had been treated so that it was safe for the children to use, even wearing gloves.
I can’t remember what it was called, but we had advice from the RHS.

You can often get broken pots from garden centres which can go at the bottom for drainage. In terms of plants, it depends what you want... Something ready made to look nice, or seeds for the kids to grow? The type of compost also varies depending on whether it will be a long term planter with e.g.shrubs, or if it will have seedlings.
Good edibles that are easy to grow would be salad leaves, radish, peas /beans, chives, parsley, nasturtium.

Will the children be around to see things grow?

Thanks for all your help! The children will be there to see them grow, but we won't have access to the garden during the summer holidays for watering etc, so we are thinking of things that are either hardy enough to survive a 6 weeks of neglect, or short-lived ones which will be all finished before the long break.

It's built into the ground, so there is earth underneath it, but it is thick, sticky clay and full of rubble so we want to cover it up asap! My concern with broken pots etc is that if the children dig down, they might have quite sharp edges - similarly I was looking at bags of hardcore online, but it looked like some of it might be quite jagged and I really didn't understand the gradings!