I visited my Grandfather.　おじいちゃんちに行ったよ。

~ ~ ~ ~ ~

Good job!

I visited my Grandfather.　おじいちゃんちに行ったよ。

I visited my Grandfather.　おじいちゃんちに行ったよ。

I visited my Grandfather.　おじいちゃんちに行ったよ。

Just one point:

1) You made a couple of spacing mistakes. This is something I've noticed a few times when Japanese people write English. In particular, there is no space between the last word in a sentence and the full-stop.

小さい間違いですが、二回スペースの間違いを書きました。日本人が英語を書く時よくこんな間違いをすると思います。特に、文章の最後言葉から句点までスペースがありません。

Otherwise, this is generally very well written and even though there were a few small mistakes, it was easy to understand what you wanted to say.

これは上手に書きました。二三回小さい文法の間違いましたが、意味は理解やすいです。

I've rewritten the piece below with the corrections plus a few minor changes I've made that aren't essential but I feel add something:

私はもう一度訂正と書きました。また、要りませんが、少し変更を入りました：

I visited my Grandfather.　おじいちゃんちに行ったよ。

My grandfather is 86 years old.
He lives alone in Fukushima.
Yesterday, I visited him with my family and stayed at his house.
He was so glad when my sister and I made him dinner.