To have numbers instead of text :
'type' => 'number',
'custom_attributes' => array(
'step' => 'any',
'min' => '0'
),
Now, gonna put all of this to trash cause what I really need is to have my custom field on each bought product : if I want to buy a bike for an adult and a bike for a child, I will have to give two different weights… o_O