I think the other insurers are also offering this at the moment.
I think it's only on their Essential Select plan that they have free child cover until March.
We are on Essential Starter and were paying 153 per month (2 adults and 4 children).
Parents do not have to be on the same plan as children - important for anyone who is worried about their own level of cover being compromised, and there are no private childrens hospitals anyway.
Rang them and they switched the children to Essential Select, left us on Essential Starter and cost reduced to 91 per month, saving 750 per year.
Essential Select is a higher level of cover but the priority at the moment is to keep costs to an absolute minimum or we'll have to give up health insurance altogether with the waiting periods and all that when things pick up.