then MrsBadger Next has solved all your problems
buy these in the size that will fit their bottom (dd wore the 2-3y from about 18m)
Turn / roll them up to the right length for their legs.
Then (this is the clever bit) snip off the button that holds the tab up
Now re-sew it at the appropriate place on the seam so the tab will hold the turn-ups at your desired length.
Done!
The best bit is that as they grow taller (but the nappy stays the same size) you can move the buttons down as required. And of course when they are potty trained and are slimmer-bummed they can wear them at full length...
If you'd rather have denim try these or these.
If you have an ITNG junkie get these.
If you have a boy you need one of these in each colour, because these aren't as nice and none of their jeans this year have button-up tabs.
If you are strapped for cash, Next do them every year so there are usually a few on Ebay.
This has been a public service announcement, thank you for listening