Excellent suggestions have been given, I have a couple more (focused on the people useful for little kids).
What kind of people (professions, backgrounds, anything) do we need to attract?
Parents who have been through Early Intervention programs generally know a LOT, and are some of the most active helpers of other parents I know.
People who work in service agencies such as Health Departments, etc. Often these people act as Service Coordinators for children in Early Intervention, and know of tons of resources (though often very localized).
Where do we find these people? How can we contact them?
Call Early Intervention programs (usually listed by county or state).
When we do contact them, what do we say?
Many of them are trying to build social network sites for their families. Yet since each county tries to build it's own social network site there just are not enough users to make it work. I'm sure many of them would love to have their families be able to access a site like SE, especially if there's a discussion of the additional functions besides the Q&A area (chatrooms, etc.).
How can we make our site more attractive to these people? What's holding them back?
Some things which might attract them:
Show them how to do private chat rooms for families. Service coordinators are generally required to contact families at least monthly with resources. These professionals carry massive caseloads and are always short on time. If they can do that with a scheduled chat time (so they can interact with several families at once), they might find that useful.
Show them how tags could be used to alert providers from their program that one of their families has a question. Though this seems like it might border on being abusive of the tag system and maybe some use of the @ symbol would be better (I don't know, I'll leave that up to you all who know a lot more about the systems than I do).
The issues I would immediately see are:
I would want my kids and families to have some way of identifying service providers or professionals from their program.
How is information protected? Our kids and families are vulnerable, I'd want to know what security is in place before I referred my families.
The answers need to be accurate and evidence based. Most of us in these programs feel overwhelmed by bad information from television, the internet, toy companies, etc. We're exhausted from spending half of our available time with our families just trying to undo the damage that's been done by people convincing them to put an iPad with "learning games" in front of their kids for two hours per day (as an example).