Do they have passports yet? If not, can you get them and keep them safe (i.e. away from your ex)
I seem to remember on another thread similar to this that passports were mentioned, you don't want him to apply for them and then whisk them out of the country when he goes for example. If you have the passports then at least he ought to be asking you for them which will give you advance notice of any plans. maybe - sorry, I don't know the ins and outs of this subject.
I also have no idea if you can flag their passports/identities to say that you don't want the father getting a second set of passports issued for him to have for the girls IYSWIM.
But definitely worth looking into sooner rather than later and also seeing if someone on MN can provide any better info!
I would also hold off for a few days on sending the email to your ex - he may have managed to work up a good sense of excitement and promised all sorts of treats and brushed over the not-so-good bits (like being away from mummy for so long, horrible long flights, etc). Given a chance to calm down and think about it and talk to your dd about why she has changed her mind, you may be able to change it back again. or at least make her more hesitant.
you're also saying you're not happy with them going - why send an email to your ex to make him happy? wait for his solicitors letter - or at the least make him stew a bit. And say that you are really not happy with them being away from you for such a long time, especially given that they still cry if they are away for just a day or two.
Could you reiterate that if they go you want to go too (at his expense of course) - just because he suggests something doesn't mean you have to agree to it (and of course he doesn't have to agree to your demands, but it puts you in a stronger position, e.g. you can negotiate down to you going until the youngest is 10 or for just the first time and you have moved a step towards compromise and he should then move a step towards you).
also could you get him to try a week with the girls in the UK before he goes - horrible for you I know but better than 3 weeks in Oz. If it goes badly then all the more reason to hold out against them going over there for 3 weeks just yet. If it goes OK then at least it will hopefully provide you with a little reassurance that they will be OK.
would also insist that it is their father that accompanies them the first time if you can't, especially if you don't feel his mum is up to it. and find out what he plans to do with them while they are there - will he be taking all the time off work - if not, who will be looking after them? And what back up plans does he have if they are poorly or he is?
finally (sorry, long post I know) is it worth playing a game with the girls about what happens if daddy thinks it would be fun to go to Australia together without telling mummy first? so that if, heaven forbid, the worst happened and he tried to do this, they would know your contact details and would try to tell an air hostess/person at the check in desk/customs officer/etc and hopefully circumvent his plans?
I'm sure plenty of others will be along with good advice in the morning...