When a child needs therapy and needs consent from the parents only one parent really needs to consent and sign the consent forms for the child's therapy to proceed.
If the parents are divorced and have custody of the child at different times then usually in some cases both parents may need to consent before the child's therapy can go forward.
But most therapists will be able to get consent from only one parent which is legal in most places so that the child can have the therapy sessions.
If the therapy for the child is deemed medically necessary then a doctor may even be able to provide the consent along with the mother or father.
Usually though if the mother consents to the child's therapy then that's usually all that's needed.