How can i explain how to make a 3.5 into a fraction for mt 10yr old?

The number on the left side of the decimal, write it as a large, in this case, three. The number on the right side of the decimal, write as a small number. Then put a ten under the five, and divide the two with a line. A way to remember this is - Imagine you have 3 cookies. The .5 represents a half of a cookie. You need to have all ten parts of the cookie to make a whole cookie. You'd need 5 more pieces of cookie to make 4 cookies. Hope this helps.