This is an interesting question, with limited information in the literature about using ReSoMal specifically in this age group. A few points from my previous clinical practice:
1. I generally avoid giving oral non-milk fluids to to infants. If they can tolerate anything orally I would prefer the usual - breast milk, or breast milk substitute if this is not possible.
2. If an infant is severely dehydrated and cannot tolerate oral fuids, I would start off with very gentle i.v. fluids (keeping an eye on that blood sugar) and re-establish feeds as soon as possible.
3. I would be very carefully about thinking of giving anything with lower sodium content (such as ReSomal) to an infant <6 months due to the risks of hyponatraemia / fitting.
So generally, the focus is on getting milk into the infant as soon as possible, following the usual guidelines, and adapting this according to vomiting / tolerance of feeds.
This is based on my experience, as I only found limited research on this!