Companion restrooms are there for people who have no other option. A singe parent of an opposite sex child, people with disabilities who need assistance, people with large wheelchairs that do not fit in standard stalls, etc. There are no "rules" but in general, courtesy dictates that if you CAN use another option, you SHOULD. However, only YOU get to decide if you can, not anyone else.
To the OP, there are a few things you need to know about Companion Restrooms in Disney parks
1. There are NO STALLS in them. This means your 8 yr old may end up seeing Mommy peeing. These restrooms are one big room, with an open urinal in most as well. There is no option for privacy. Every family is different, but not all would be comfortable having potty time being a group activity.
2. The Companion Restrooms are NOT on a standard park map. You have to ask for the special map for Guests with Disabilities (which is different and not kept at the turnstiles).
3. There is not a Companion Restroom for each regular one - there are only 5-6 in each park, and some are in locations fairy inaccessible (one is in Cinderella's Royal Table, for example). If your child needs to go potty RIGHT NOW, it may well be a 15 minute walk to the nearest Companion Restroom. Consider options for 5 yr old bathroom emergencies.
4. You cannot see the outside at all from inside the Companion Restroom, which means all children will need to come in with you every time. Also, the door is usually thick fire door, which means they cannot hear you and you cannot hear them.
5. Many people who use these restrooms take quite a long time to complete toileting activities. Therefore, do not be surprised if you have to wait 20-25 minutes for the person in front of you to finish. The other moderator even had a guest call Security because it takes a while to help her daughter (full time wheelchair user) use the toilet.
6. The toilet will be a raised toilet with handrails, which means it may be quite high for children to sit on comfortably
I do not say this to make you avoid the companion restrooms, but so you know the limitations they have. If you cannot use another restroom, then by all means use the companion restrooms. I just like people to know that they are very limited in number and that they are the only option for a lot of people visiting the parks.
Personally, I would not think twice about an 8 yr old in the ladies room. I am at DL all the time, and I see 10-11 yr olds in the ladies room all the time!