Writing is a tricky skill! Children have to:
* think of what to write
* break sentences down into words
* break words down into sounds
* think of how to write the sounds/words
* physically form the letters

When practising at home, try to make the writing as purposeful and fun as possible. You could try:
* writing shopping lists
* writing to relatives (or to me: you could write a letter, take a photo and email it to school. I will reply!)
* drawing a cartoon strip and writing captions
* making posters to go up around the house
Fine motor skills and letter formation

Both YR and Y1 children need to build their fine motor skills. Science has shown that children's hands are not actually fully formed until they are seven years old (there is an image in the downloads below, comparing 4 and 7 year olds' hands). You can do lots at home to help your child's fine motor skills: see the download in the links above. There are also letter formation sheets. We use the Twinkl families at school so children recognise the similar movemements needed for, eg, the 'caterpillar' family or the 'robot arm' family. You do not have to practice letters using pencil and paper (though some children prefer this) - you can practice with pens, on walls, on a piece of paper taped to the underside of a table (lying on backs and writing is good for muscle development!), outside using chalk, with fingers in a tray of paint.... there are many possibilities. 
Once Upon a Picture
This website is full of interesting pictures. You could choose one, print it off and write a sentence (or more, Year 1!) to describe what it is showing or what is happening in it. 
Click here to go to the website. 
Reception: what to expect

Your children are, of course, all at different stages with their writing development. Over the next few weeks, focus on letter recognition, letter formation and fine motor skills development as a start. 

Then you could help your child to write single words: if they have built a lego model, for example, you could help them to label it. 

If your child is ready, they could write a single sentence eg 'This is my bedroom!' to go on their door. Help them to hear the different sounds, find them on a sound mat (download above) and write them.

A good way to break these skills up is for you to write the words while your child tells you the sounds, and then for your child to copy what you have written.

They key with writing in Reception is to make it purposeful and fun. 
Year 1: what to expect
Each child is, of course, at a different stage with their writing: but here are some general guidelines.
Children can write simple sentences with spaces, capital letters and full stops. Do encourage independent sounding out for writing and do encourage children to use the spelling lists ('colourful spellings' above: lists 1-4 have been covered) to correctly spell 'high frequency words'. 

Do encourage correct letter formation.

Don't expect perfect spelling: phonetically plausible words are okay at the moment, with some spelt correctly. 
Your child could keep a diary of each day to show me: they could write simple sentences (with finger spaces, a capital letter and a full stop) or they could use a conjunction to extend their ideas (we have briefly looked at and, but, so and because). They could make up a story and turn it into a book. They could write a letter or a postcard to a friend or to me (a photo of this could be sent to the office email address).