Kids shoe measurements

[Kmoggy]

Ok I'm so confused. I been to Clarks and another store who say my kids feet are 3.5 &4. I don't want to buy from them as I don't like hat they have so I came home and measured myself. Looks like a 20 /21 in the shop I want to buy from but I have wee cheap h &m shoes upstairs and they are miles too big for them and those are 18/19. How on earth do you find what size your kids feet are when this young? As everywhere has different sizing

I should note was told 3.5 in Clarks with a foot length of 13cm. But looking at shoe chart 13 cm is 4.5 uk. Do Clarks measure differently?

Some brands come up a bit smaller, some are narrower so you may find yourself going up half a size & vice versa.

clarks seem to measure small.
tbh I fit dc's shoes myself, if you are unsure there are youtube tutorials.

I didn't realise it would be this hard. I'll order from bobux in sizes I'm measuring and take it from there. Thanks

Does anyone know how Clarks sizes compare to Birkenstocks? Tend to find Clarks are large compared to high street (Next or Asda etc) and want to order some Birkenstocks online (currently 11G at Clarks)
Thanks