Here's the thing.
If they didn't want kids to play with it (it's like anything special, the rule is - and you must have similar thing in your home) they could put it out of sight.
We have Lego sets our son has built that are very special to him - the huge Harry Potter ones, etc. He has them displayed on shelves in his room. He used to actually not have certain friends over because he knew they'd be rough with them. Or they just don't go to his room. That's fine.
I'm not saying your kids were rough, but if the Lego was brought out to be played with, then your kids may just have been 'playing' with it (not roughly) and a few pieces might have come off. All it takes is a few to break off, and it's hard to fix those darn sets.
Anyhow, we've had kids (almost all) come to our home and rip heads off Barbies, Monster High dolls, break dollhouse pieces, furniture, you name it. I've had pool toys get destroyed .. it happens. Kids are kids.
Honestly, I think that sucks she texted you. A call might have been better. It's awkward. I've never once reached out to a parent anytime something was broken. The most I've ever done when my child came to me to let me know a toy was broken (what I've asked them to do) is say "now girls, what happened" and the kids tell me and I just say "Ok, let's just play a little bit more carefully" or I'll say "How bout I put these away for now and let's take these out" or "snack time" or something.
I never make anyone feel bad, because honestly, it's my mistake for leaving a special toy out (with the kid) if I don't know if a kid is going to treat it with care. If it's that special, we put it out of sight until later.
Just apologize if your kids actually broke it, and that's that. It happens. What are you going to do? Offer to do it all over? I mean, I suppose you could, but to me, that seems really extreme. I personally, would not. I would just say - they are sorry, and didn't intend to break it or the girls could.
*if they don't have the instructions - you can download online these days