Flowergirl dress stress

I thought I was being super-organised and bought all 4 of my flower girl dresses and BM dress in the BHS sale in August. Bargain - £112 for all 5! I pre-empted the fact that children grow rapidly and bought all the girls dresses 2 sizes bigger than they needed, with the exception of my 7 year old niece who I bought 3 sizes bigger for because she's always been a well built child. I was assured when she tried it that there was plenty of growing room both up and out the way so we're all happy. I rang my sister the other night to enquire about shoe sizes believing with 3 months to go we should just about be able to gauge what we'd need to get depending on how recently a growth spurt had taken place. Oh yes, she's just been measured, her feet are currently a 12.5, a 1 should be fine "by the way Fee, she's also grown quite a lot since you last saw her". This is good I think, no need to shorten the age 10 dress as much as we thought. I thought no more of it. Tonight my mum tells me that height isn't the issue, it won't do up, not at all, nowhere near. It's ok I think, I've seen hundreds of them on ebay and preloved, I can pick up an age 12, she can't outgrow that in 3 months surely? But no, not one over an age 6 to be found in any design in the colour we need. I can find the same dress as my BM in a size 10 or 12 which we could alter but it's too old for a 7 year old. I can't track down a bolero in the right colour so we can cover up the back that won't close. I can't start again in the search for dresses so my option is either to put her in a different dress, find a way of making the adult dress more suitable or see if we can get a lace up back put into the one she has. What would you do? Feeling a little sorry for my niece who's feeling a sad about the fact she currently has no dress.