Seriously, don't fret!
Particularly if they are in Reception and are young for their year.
My daughter is now 5 years old, as of last week; and she is writing well even if the teacher continues to criticise her method of writing (she likes to start at the bottom of letters, not the top. I'm a big believer in results, not method, so can't see why this is a problem), and she is reading well, struggling only with naughty words like "was" and "here" which don't sound like themselves at all when you try and sound them out a letter at a time.
She's been at school now since September 2010. 8 months. Her sister won't start school until a week before she turns 5 years old, and therefore, in order for her to be anywhere near the same stage as her sister as at her 5th birthday I will have to have taught her all the stuff that school have been teaching my eldest for the last 8 months.
And so, when the teachers compare my eldest's performance (one of the youngest children in her year) with the oldest children, I am frustrated, as the developmental differences between children of 4 and 5 years old is vast, and yet the teachers seem insistent on them all being the same.
Don't worry. Don't fret. Reading and writing are important. But they are only young, and have plenty of time to work it out. Plenty of time.