The book recommended is a good insight for Mum to understand what maybe going on. Also a rush of hormones at seven and eleven may make some children susceptible to feelings of extra anxiety, or they suddenly are pushy/shovey, testing the boundaries, acting out etc etc.
Well done to Mum for spotting this, it may make the child feel odd and different and unique at the exactly the time they just want to be the same as everyone else - gotta love biology.
I have three of the little darlings two have emerged from "the dark years" one to go - but now they are twelve we are in back in the same boat. Just be honest and reassuring where possible citing facts from history to back up what you are saying. My friend's son became fixated with all current war news stories and it was a tough time. Pre-empting what you think they might ask you can help you to have a satisfactory answer to hand, or don't be afraid to say I have honestly never thought about it - can I get back to, but make sure you do, kids rarely forget. If they don't get the answer from you they might seek it from their peers - and that can mean you end up in very interesting ( but not usually fact based ) territory. Also I have bought some of those little worry dolls that they have in Oxfam for when my daughter just wants to say something out loud and then forget about it.