I'm new here and new to cloth nappies, so I may not know what I'm talking about, but I was wondering whether you had tried the Disana tie nappies?
I've found them at Cuddlebabes.co.uk and Twinkleontheweb as well. They're a good price especially if you already have things you can put inside them to boost them with.
They aren't meant to be used alone, especially not for heavier wetters, but they seem to be very generous. I can't imagine them not fitting an older toddler, as they are very stretchy knitted cotton and have to be folded down quite a bit for a newborn. You can add any inserts/boosters/prefolds to them you want, and then cover the whole thing with whatever roomy fitting cover you have to use (as it turns out quite bulky when boosted a lot, but so, so soft and cute).
I know that some mums aren't keen on the idea of using nappies with ties, as they prefer poppers or velcro, or nippas even, but the Disanas do seem like the most likely to fit right through to potty for taller children.
have you looked at the Easy Peasy Bumper nappies? They offer them for bigger children, and they do them in bamboo now, as well.
I found a thread on forum from the US where some ladies were discussing these, and some posted pics of the nappies in use with different aged children.
if you scroll down on the page above (assuming the link will take you to it, lol), there's a cute photo of an older girl wearing one of the nappies.