My understanding of the Family Law Act and in particular S65 DAC and DAE is that decisions that have no long term effect such as a doctors visit for a cold would not need both parents to agree. If, however, your intent was to take your child to a paediatrician to have them diagnosed with a ongoing medical condition and be prescribed with ongoing medication (ADHD and the like), this would require agreement from both parties.
If you believe there is some reason for your child to see a paediatrician and both parties can not reach an agreement, its best not to try to back door the other party. They would likely have grounds to establish a breach of the parenting orders. It might be best in this case to seek to vary the parenting orders or have the court agree on your proposed action.