Ds2 has a tiny waist but long legs....where can I get trousers to fit?

[nobodysfool]

Goodness knows where he gets it from but DS2 (just 4 years old) has a tiny waist and long legs and I am struggling to find jeans that fit.
I would prefer jeans/trousers with the ribbed waist band so they are easy to pull up and down. He needs aged 4-5 for the length but they just hang off him.Any ideas where I can fin small waited bottoms with regular leg length?

Ds1 is the same. Zara trousers fit him well. Also a range in house of Fraser called 'name it'
Gap slim fit/skinny chinos

Petit Bateau, Benetton and Boden all slim fitting with adjustable waists.