Hi Keeley, welcome to the forum. You obviously like wildlife, otherwise you would have just filled it in and killed the newts.
I understand where you are coming from about the pond, but I too would try to discourage filling it in. I did a garden pond survey near me and it was frightening the number of people who had filled in ponds becuase of small children and then never replaced them when the children had grown.
My suggestions would be, depending on the size:
1. Never allow the children or their friends in the garden unsupervised.
2. Enjoy looking in the pond with the little ones, but tell them never to go near alone.
3. Put up an unclimbably fence around the pond area.
4. You can buy ready made pond covers, but these can be pricey. If the pond isn't too big, I would make a wooden framework from planks to fit over the pond. Make a similar separate framework. Buy some sturdy plastic or metal mesh- you may find suitable sized plastic coated fencing panels.
Stape the mesh to the top of the first frame, then screw the other one on top. Obviously if the pond is a big one, then this might be too flimsy, but it will definitely work for a smallish pond. The mesh needs to be above water level.
5. Teach your children to swim, and teach them that water is dangerous if they go near it alone.
Could you upload a photo, so we can help better?
Ask around your neigbours to see how many ponds there are nearby.
On a slightly different theme, there won't only be 40 newts in your pond.
It sounds as if there is a high population which also includes eggs and efts (tadpoles) in the pond at the moment, and all the newts living terrestrially as they only live in the pond when breeding, and young ones live entirely on land for the first 2 or 3 years.
...I have just looked at the Rospa link- the bit about the mesh sagging. There are some proprietory grilles which are below the water surface. This is no good because if a child trips over and bangs its head, their face will be under water.