I doubt that the recption kids will be 'reading ' much at the moment. Although you never know !!
discuss the book with them and the pictures . Let them say what they think the story will be and what the pictures tell them.
get them to point to the words whilst you read them if necessary. If they ahve been doing phonics ask then to pint out certain sounds that they see i.e SAT PIN E CH CK.
Lots of encouragement and praise.
I did parent helper last year with reception , although didn't do reading but have DS1 in Yr1 and DS2 in reception .
If you have a child in reception tell the child that you are reading with that you are for example, 'Emilys Mummy' otherwsie they may be a bit bewildered as to who youn are and dont spend too much time with youn own child.
Don't forget to write a comment in their reading record book . Good Luck .
Our school will not have parent helper with Reception until after half term to allow them to settle in.
You'll enjoy it and will see children of a smilair age at lots of diffrent levels.