We typically use a regular refractive index detector to detect oligosaccharides in this range. If you use an amine-based column, separation is straightforward. With a C-18 column, it is possible to see double peaks due to separation of the alpha and beta anomers of reducing sugars.
Out of curiosity, I would wonder why they cannot be detected by ELSD????
It is certainly possible. Go to the Waters home page (waters.com) and download their BEH Amide applications book. You will find several examples including oligosaccharides in the mass range you are interested. Most of the examples are for UPLC but you can do the same on HPLC using columns with 3 or 5 um particles