Plenty of families are in receipt of DCA for more than one child so that won't be the reason. Its most likely to be because whilst they accept the child has a condition or disability, they don't think it is severe enough to justify DCA. A most important thing to mention in the appeal is precedent i.e. that your 8 year old is in receipt of DCA for the same condition. You will need strong medical evidence from everyone involved in the management and treatment of the childrens' condition i.e letters from doctors, consultants, speech therapists, OT's and therapists outlining the condition, treatment, medication and their support of your application, if appropriate.
Also mention the various appointments the children attend and the regularity and any costs incurred... Good luck.