Learning about other's beliefs

After the atrocities in Paris a pair of pleasant muslim men came to the door offering to leave me a couple of booklets about Islam to try and counteract the anti Islam publicity. I found the literature very interesting (nothing derogatory about women at all) but I did wonder what sort of welcome they would get from some of the neighbours.

I am sure staff would be with the children

I agree with Luckygirl that the decision is for your son and d-i-l to make. On a personal level I would be uncomfortable if an Imam were to speak to young children unless there were adults (grandparents and parents perhaps?) present who would challenge - really effectively challenge - any religious dogma that flies in the face of our society's values, such as equality for women. On a practical level the school could always ensure that the Imam is endorsed by the Quilliam Foundation which promotes integration and a moderate and inclusive Islam.

It's a pity, the Iman is reaching out to the community

I think the Imam would have been asked about the content of his talk to the children before it happened.
As I understand the rank and file Muslims hold respectful views of women and hate the extremist views that have taken hold. However it is the parents decision to make, did they talk to the head teacher about their concerns?

Oh dear will the children know why they are not in school that day?

I think it is their decision to make, as you clearly recognise.

Learning about a diversity of religions can only be good, but I guess it is important to know as parents what this person is going to say and which version of Islam they hear. Many Muslims will tell you that women are respected and honored; others regard them as second class and want them to walk behind them and be covered up. I can understand that your son and DIL are not keen for the children to get a message that downgrades women, so, although learning about others and their beliefs is in principle good, I can understand their anxieties.

Frankly I too have no idea what the "real" Islam thinks about the place of women - who is to decide what is "real"?