This source might not be so reliable because it doesn’t say who took the photograph, and by all the kids, everyone of them happy, it appears like they might have been asked/bribed to look so happy in order for a photo to be produced to encourage parents and children to evacuate the city and be “safe”
So maybe it was made as part of propaganda by the government, to tell/show/ask parents to send away their kids to the safe countryside.
Source C is an interview which was done in 1988 with a school teacher that took part in the evacuation. (Immediately you can notice that it was done well after the photograph was produced.
From the interview we can see that the school teach took note of the fact that some mothers were very upset, some ok with it in the hope f that they would see their child again, and some mothers casting away their child.
This source seems reliable because it is an interview with someone who was there at the time, so instead of a photograph which could well be fake, the interview seems to be true and real in the fact that, why would the teacher make up a story or lie about things which have happened?
But it must be taken into consideration that the interview was take 49 years after the photo so he/she might have forgotten something’s about what happened.
Its purpose of the source appears be to show how the evacuation of the child of England/London didn’t run as smoothly as people were made to think and that other things were happening behind the image that people were made to think.
It doesn’t show how some of the children saw the evacuation as an adventure/trip/holiday and that not every child disliked it, Which is a weakness in this source.
I think that both of the sources are useful in their own way.
Source B (photograph) shows how the children enjoyed being evacuated to the countryside. Or at least that’s what the photograph was made to look like. And Source C explains how the children/parents acted towards the evacuation.
Both sources could be fake, but I believe that both of them are real to a certain extent that it gives an impression of what certain people wanted you to think about the evacuation.