Safe is not determined by the website hosting the e-mail account.
Now did you mean safe as in safe for the computer, or safe as in his/her personal safety?
Safe is determined by how you supervise the account (for both the computer and him/her), whether or not you click on links in an e-mail (mainly for the computer), whether or not you open spam messages or messages from an unknown person (usually doesn’t hurt the computer to read a spam message on a web based service, but might be unsafe for him/her).
I use yahoo mail and am very satisifed with it. Some like hotmail, some others. Any common web based e-mail is just as good as any other in most respects. If you want him/her to be safe, you have to monitor the account as well as how they use the account. This is for both computer safety and personal safety.
Oh, and no matter who you use, you’ll get spam. Most spammers randomly spam combinations of letters and numbers to hit most possible e-mail addresses. Which is why you can get spam within a day of opening an account.
So it’s parental involvement that determines safety, that should be your primary concern if you want your kids to be safe.