Sunset that day (19 February) was at 17:26
Fit adults doing Scafell by the quickest route typically take 5 hours to go up and down again, if they are hurrying (e.g. as part of the Three Peaks Challenge).
A party involving a five-year-old could reasonably expect to take quite a bit longer than that (this is not assuming a "townie" 5yo who has never seen a mountain, this is assuming a 5yo who is smaller and slower than a very fit bloke in his 20s. And in this case the family were from Cornwall, an area with many great walking opportunities but no actual mountains.)
Therefore if they set off at midday they could not realistically expect to be down again before sunset, even if they didn't stop at any point to have something to eat.
The path down Scafell is notoriously (a) hard to find, and (b) slippery underfoot. This is not good if the light is beginning to fail. There's no indication that they left any word with anyone about where they were going or what they were doing, and there's very little mobile phone reception on Scafell.