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roomier clothing sizes
I generally find C&A clothes to be 'roomier' than equivalent sizes at say, H&M or Zara...
Have a look at Gap online. Without being funny, they are designed for the US market and are often a bit roomier than some of the EU ones. Delivery is cheap and can be free.
http://www.gap.eu/browse/home.do?gatewayPassed=GP_EU&preferredLocale=en_...
Laredoute.be has kids trousers with three different fitting under their own product line Vertbaudet (search with 'morphologik').
You could try Esprit
http://www.esprit.be/enfants/nouveau-boutique-des-basiques/petites-filles
Agree with poster about ordering from Gap online and you could try Lands End too (UK site delivers to Belgium)
http://www.landsend.co.uk/Kids/ix-Tab_17?cm_re=kids_tab
Some UK sites may be of interest:
http://be.nextdirect.com/en/
http://www.boden.co.uk/en-GB/Girls-1H-12yrs-Clothing.html
http://www.marksandspencer.eu/en_BE (or visit the shop at the Toison D'Or)
Good luck!