Hi Kev,
Ravenspring
www.ravenspring.co.uk ( Better declare that we are their Irish Agents) will make you up kids Dry suits which should ( because they are kids clothing) be VAT free.
They also have a lot of experience ( because they had kids themselves!) of knowing how to build in gowing room so that you can get max value out of your purchase.
Talk to Ann Morley. You'l find her very helpful !
Best wishes,
Stuart