Q: How to separate 2 IPADs using the same Apple ID

I'm not quite sure that I understand what you are talking about. If you have two 16GB iPads, they only have about 13 GB of available storage out of the box. Using the same Apple ID on both devices does not impact the amount of space that you are using, unless you are syncing identical content to both and have automatic downloads turned on for everything, and are using iCloud Photo Library on both devices.

What is it that you are trying to achieve? What exactly do you want to do?

Firstly each iPad will never allow you to store 16GB in total because the OS takes up some space on the storage. There is no way to avoid that. Some iOS versions are a bit more space efficient, but you never get access to all of it.

If you have the App store setup to download all purchases each device will use space for that, turn off that option in settings or delete any apps you do not want on the device in question.

If you use iCloud to sync documents & photos you will also consume space. Creating a new Apple ID may help, however you end up with data in 2 different places and can easily get into a mess. It may make more sense to decide what data is actually important to your daily usage & remove any that is not important. For example you may only want all your photos to be on one device.

If you have many GB's of data syncing from iCloud you will have issues, iCloud has very little control over what is copied to the devices, so expecting iCloud to store too much data can lead to issues like this.

I'd suggest you look a 'Settings > General > Usage > Manage Storage' to see where space is used & then ask for help to clean up those areas of the system if you need it.

Settings > iTunes & App store, disable the automatic downloads of apps & anything else you want to manually add.

You should be able to delete apps without effecting the other device.

If you are certain you want some separation you could create a new Apple ID for the kids iPad, however that can cause them to lose game progress & probably require you to purchase apps again, otherwise you risk merging the data again if you use the same ID. Eventually they will want their own Apple ID, which will mean they lose progress & purchases.

Ok. You want to do as I suggested by turning off automatic downloads for starters. Go to Settings>iTunes & App Store>Automatic Downloads>Off for apps, books & songs. That will prevent whatever the kids download on their iPad from getting into yours and from yours to theirs.

Then go through each device and deem what is important or what is needed and delete the content that isnt important on that device. You can check which apps are taking up the most space in the settings. Delete apps that you don't want.

Check your storage on your iPhone, iPad, and iPod touch - Apple Support

If you have photos on the devices, they can take up a fair amount of storage, so think about importing them to your computer for safe keeping and then delete them from the iPads.

Import photos and videos from your iPhone, iPad, or iPod touch - Apple Support

Movies & TV shows are storage hogs as well. You can delete previously watched videos and download them again if desired using the Apple ID that you used to purchase them.

Download your past purchases - Apple Support

You might want to consider setting up family sharing as well.

Family Sharing - Apple Support

You're welcome. Good luck moving forward.