You can upgrade any family member to Kagi Ultimate plan for $15/mo and it will give you unlimited access to GPT-4, Claude 3 Opus, Gemini 1.5 Pro and other state of the models.
Yeah, they don't advertise it very well. It was actually a pretty good value add for me; I was already paying $10 for Kagi search, and $20 for OpenAI, so the $25 for both actually saved me $5/month.
I think you can upgrade to Ultimate at a per-account level for another $15/month.
EDIT: Accidentally wrote "pro" when I meant Ultimate.