Children with Down syndrome will vary widely in their achievement of numeracy skills, and the small amount of research in this area suggests that their numeracy achievements are at a lower level than their literacy achievements.  However, when a student is motivated, taught well and participates in errorless learning it is possible  for that child to achieve at an age-appropriate level.

Resources

Bird, Gillian and Sue Buckley  Number Skills for Individuals with Down Syndrome - An Overview

                                                      Number Skills for Individuals with Down Syndrome (5-11 years) 

The Clarke Road Money Programs Kit

Horstmeier, DeAnna  Teaching Math to People with Down Syndrome and Other Hands-On Learners 

Squire, Anne  Maths and Money