I have bought some really nice dk yarn but realise it's meant for 4mm when - apart from the ribbing, the pattern is for 5mm needles, I should have bought Aran weight.
So if I were to do this yarn on 5mm, its going to be rather lacy and loose. On the other hand a lighter/lightweight jumper might be worn more than a thick one.
What will happen if I knit it on 4.5 instead? I can see it might mess up the shaping slightly - but it's a basic child's sweater, not a designer garment.
www.berroco.com/sites/default/files/downloads/patterns/Berroco_FreePattern_Lawrence.pdf
It also wouldn't matter if the end product came out a bit small - providing I get the neck wide enough - as it's for a chunky 2 year old, and I'm opting for the 4 year old size....