That's not possible. One field = one data point. If you want three distinct data points, you need three fields. Children is a keywords field, which means that it accepts multiple values, separated by commas.
So, either use the Children field as it was designed to be used or create three new custom text fields, to store three different children.
FYI, there is a newsgroup specifically for Outlook forms issues "down the hall" at microsoft.public.outlook.program_forms or, via web interface, at http://www.microsoft.com/office/community/en-us/default.mspx?dg=microsoft.public
.outlook.program_forms

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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
http://www.turtleflock.com/olconfig/index.htm
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> when designing a form for contacts, how do i use the same field more than
> once without having the values automatically repeated ...
[quoted text clipped - 5 lines]
> please advise how to make each "children" field able to store its own
> values/names.