I have my DHCP server issuing Internet Guide DNS servers to all the clients on my network, which works fine... I've created a second security plan (one for adults, one for my kids).. but it seems that you can only have ONE security plan active at a time...

... is there a way I can somehow configure my kids' PCs to use IG with the kids security plan, and the adults' PCs to use IG with the adult plan?

No - Internet Guide is on a per (WAN) IP basis. You can configure the adult's PCs not to use the IG DNS servers, but that's it.

OpenDNS offer a bypass option in their Enterprise level service, but at a starting price of $2K/year I doubt you'll want that Later this year they're apparently bringing it to their Deluxe package - a much more reasonable $10/year.