It is important that parents help with brushing teeth until at least age 8. Before this, children do not have the dexterity to brush well.
It can be easier to brush if you have your child lie down with their head in your lap. Then you can see into their mouth and see what you are brushing.
Consider moving tooth-brushing activities out of the bathroom into a common area like the kitchen. Then, your children can brush as a group while you monitor. This helps you to make sure they are brushing instead of just playing.
Set a timer for 2 minutes, to make sure that brushing happens for an appropriate length of time.
For the independent child, allow them to brush first and then you can brush. This practice allows them to develop their skill.
If your child cries with brushing, don’t give up! Continue to work on this habit daily and usually children become used to the routine.
Start when children are very young. It is easier to get a child used to tooth brushing if you begin the habit from the very moment they have their first tooth – usually between 6-12 months of age.
Ask another adult to help you. If your child really struggles, it can be useful to have one parent hold the child in his lap while the other parent brushes the teeth.
For children who really fight brushing, it is important to eliminate all sugary foods and beverages and see a dentist regularly. Brushing is the best way to prevent cavities. If you are not able to brush well, or at all, it becomes more important to use these other ways to prevent decay.
Allow your child to hold a comfort item, like a prized blanket or a favorite toy, while you are brushing.
Children with autism often benefit from a specially designed three-sided toothbrush head. The design allows the child to chew on the brush rather than brushing, which can result in increased removal of plaque over a one-sided toothbrush. One brand is the Den-Trust 3-sided toothbrush.
Here is a great article with some more tips https://parenting.blogs.nytimes.com/2014/10/20/for-some-children-with-autism-even-a-toothbrush-is-a-challenge/?mcubz=1. Also HealthyChildren.org tips.