Telling older kids about multiple pregnancy

I am currently 9 weeks 1 day into my 3rd pregnancy - first with twins. I had an ultrasound at 7 weeks 4 days that showed two fetuses in separate gestational sacs, each with a heartbeat. I have two older kids who are 7 and 9. My original plan was to wait until after my 12 week ultrasound (on April 1st) that we're having twins but part of me wants to tell them I'm pregnant after my next appointment on the 24th when I hear the heartbeats and then tell them I'm having twins after it's confirmed all is progressing after the ultrasound.

Any thoughts? Anyone have experience sharing with kids in that age range? I'm really nervous about how my seven year old will react but I spoke to our pediatric therapist and she said as long as we frame it positively and as an exciting thing and follow up with making sure they feel loved things should go well.