The normal rule is (as from this April) that claims can only be backdated 1 month. However, if you are waiting to hear about a decision on a disability claim such as DLA then the backdate can be longer if you notify Tax Credits within 1 month of getting the decision.
There are different circumstances. The above applies if you are starting a new claim for Working Tax Credit, but generally relates to the claimant or partner making the DLA claim for him or herself. Something similar applies if you are currently claiming & a child who is already on the claim (i.e. you are getting Child Tax credit) is the person for whom the DLA claim is made. However, you do need to check the exact requirements with the Helpline. There is no harm in ringing them now & explaining the situation. They should be able to tell you what will happen if DLA is awarded.