The parents should consult with their child’s doctor to find out if the child can tolerate a low-gluten host. If so, the Church permits the use of low-gluten hosts for celiacs and the child can receive Communion under the form of bread. If not, the parents should discuss with the doctor the possibility of the child receiving the precious blood. While I am not a doctor, nor this child’s parent, many children do receive Communion under the form of wine on a regular basis and are not harmed by the ritual sip from the chalice. Even a very small sip of the precious blood counts as a full Communion. If the child cannot risk contamination of gluten from the host, a separate chalice of wine can be consecrated especially for her.
If this child cannot receive a low-gluten host and if her parents – due to concern for her health – refuse to allow her to receive from the chalice until she is older, then she must abstain from the Eucharist until she is old enough to make her own decision to receive the precious blood. It will be a trial certainly, but one she can offer up in union with the suffering of Christ and the sorrow of his Blessed Mother.
As for the faith question, although the substance of the bread and wine is changed into Jesus Christ, the appearances of bread and wine (including their properties of gluten and alcohol) remain. So, yes, these parents do have to be careful to accommodate their child’s allergy to gluten and her ability to tolerate alcohol when approaching the question of how she will receive Communion.