I have just knitted a stripey jumper in baby chunky wool for my grandson. My query is; do I sew it together with chunky wool or use a thinner one?
That is a question of personal preference really.
Normally I would use the same yarn for sewing up a garment that I used to knit it but sometimes, as in this case, it makes sense to use a finer yarn.
Depending on the age of your grandson, it might be better to use a finer yarn to keep the seam size to a minimum. Some kids have issues with seams rubbing or irritating their skins. It would also be easier to join the seams with a finer yarn that the chunky one.
If you are using an invisible seam
to sew up the garment, you will not be able to see that a different yarn was used, but I would still use one that was a similar colour just in case.