If your child likes writing at home, then let them have a go. They are not limited to one sentence, if children want to write more than one, or even a story about it. This should be encouraged! When children are writing, we expect them to use their phonic skills to write unknown words, but 'tricky words' such as to, be, me, the etc are expected to be written correctly.
As wrong as it may feel, a typical sentence from a Reception child may look something like this...
Jack went up the been stork and sor a big carsll.
This is fine and should be encouraged. There are Phase 2 and 3 sound mats on tapestry if you haven't downloaded them already. This helps them if they can't remember how to write a sound. Sounds made of 2 letters are called digraphs and sounds made of 3 letters are called trigraphs. They are familiar with this language.
Letting them write like this helps them combine the skills for writing better than just copying your writing, or being told the letter to write. If they are at an early stage, they may just write strings of random letters, or only the first sound, and that's okay too.