Speaking from the perspective of being the DC (I was slightly older than your daughters but my siblings were the same age as some of them), my mum just started by relaying what the consultant had told her and then that there wasn't anything else they could do and that she was going to die. She cried, we cried. Then we sat and hugged for a while.
One of the things that helped was her telling us we could ask any questions even if they were things we wouldn't normally have discussed (like finances). We had one-to-one conversations about our fears over the days after the initial news. She asked us if there was anything special we wanted to do together.
One of the things I regret is that some of the things my mum wanted to put in place didn't happen because she delayed sorting them out as she couldn't face making it all real by doing them (understandably) but then things accelerated faster than anyone expected and it was too late to sort them out - which was distressing for her when she realised she wasn't going to be able to get it sorted.
I think it would have been worse for us if she hadn't told us as soon as she knew. It meant that we could at least make decisions knowing the implications rather than looking backwards afterwards and tormenting ourselves with "if I had known we had limited time, I would have done X and wouldn't have done Y".
I still do that sometimes about the lead-up to the "news". If I learned that she had known and didn't tell me, I really would have struggled to come to terms with the guilt of feeling I hadn't made perfect decisions / been a perfect daughter in the time we had (whereas it's slightly easier to forgive myself for the period before we knew). I had already worked out that something was very wrong in the hours before she told us, I'm not sure how secret you'll truly be able to keep this.
It's horrible news however you deliver it (both for you as deliverer and them as recipient). I don't think there's a time or a way for it to be non-horrible - but that's not something you're causing and there will be non-horrible and nice moments again after the news is shared.
My mum had her siblings with her for support when she told us. If you've agreed with your siblings that they'll look after your girls, maybe they could be with you/nearby when you share the news? To support you, to support the girls, and to provide visible, physical reassurance that they're going to always be looked after.
I also would be very clear and direct about what you're telling them. When my mum told us, I didn't want to believe it and with every word she said I kept trying to tell myself it wasn't true and there was something else that could be done. It was horrible to hear it confirmed that I was going to lose her and that this was really happening, but I needed to have it clearly and unequivocally confirmed so I wasn't left with false hope or misunderstanding what was happening and therefore missing the opportunity to ask her things and share things with her. It was an awful moment but it would have been so much worse if I had come away from that conversation believing she could be cured when that wasn't possible.
We didn't have any support outside the family which made things worse - I'm glad people have been able to recommend places that will provide support. It would have helped us.
I've waffled a bit, but I hope some of this makes sense.