small kid's bike

[mig]

i'm helping to get a little local girl to get confident on her bike on the quiet evening roads. alas the (new) bike she has is too small.

http://www.argos.co.uk/product/2445539

on the second ride i raised her saddle maybe 5mm or so and she instantly found pedalling that much easier - especially uphill. there is no mark for the maximum height for the post and not a great deal of post in the frame anyway.

what would the minimum insert typically be to be within guidelines? i haven't really dealt in children's bikes for a few years. would there be any scope in sourcing a longer seat pin as funds don't allow a bigger steed at the moment.

thanks!