Really sorry for you and your children
I lost a lovely mare a couple of years ago with this. Like yours she seemed a touch colicky and the vet treated as such and in my experience I expected her to improve but she didn't and I called the vet again mid day and we were still going down the colic route but something didn't quite add up. Most of the day she was very quiet and wandered a little, got up and down but you would have expected a colic to have cleared with the treatment she had or escalate and nothing much changed all day. Then suddenly in the afternoon she jumped up and sweat just poured out of her and she seemed to go into shock. I called the vet and we had both by then decided it wasn't colic and he had spoken to another vet and by the time he arrived we were both thinking grass sickness. She had calmed a bit but clearly she was not going to make it and we had her PTS.
In the following days a friend lost her stallion and we heard of another locally.
It was following a wet cold period where temperatures didn't get into double figures and it was April time. It has been drier and warmer here but there is a lot of poached ground so that may be a factor.
Walking a horse with suspected colic is viewed as a rather old fashioned treatment now and actually may do more harm depending on the type of colic.
As I said really sorry for you and your family.