We have discovered a number of friend's ISPs limiting the number of email addresses they can have to one. BT Business 500 is one of these-currently charging an extra £10 if you want more than one email address or want to use a non-BT email address. Others simply don't permit it.
As we are all family people with children who want their own email accounts (to which I apply a very high level of filtering), I am not sure how we cope with this. For eg, I work from home one day a week, working online and picking up my work email. If this is stopped, I will not be able to work at home, as I currently can't afford to increase the monthly payment of £30 to pay for extra addresses.
Many hosting companies will allow you to have multiple POP3 mailboxes, and even if they don't, all mail addressed to anynameyoulike @yourdomain will arrive in the mailbox. As you say that you "apply a very high level of filtering" to your children's mail in any case you could simply filter the mail from one POP3 box via Outlook or Outlook Express. Everyone would get their mail in separate folders and all you'll need to do is pay a renewal fee for the domain name every couple of years and a small charge to the hosting company.
One advantage of doing this is that you are free to move from one ISP to another without the hassle of changing email addresses - you'll have the same address for life.
I've posted this link before- click here -but for a different reason. have a browse, pick a free to sign up isp so you get an email addy, and set your mail application to check it from your default connection. repeat as often as required.
dipsy is correct, once your local excahnge is ADSL enabled may may use any ISP. Before you sign up though it's worth taking a look at click here where you can check general performance levels & prices, happy hunting.