Every SMS I send from my iPhone is sent as an iMessage

No. Even if you turn OFF iMessages and send an SMS then both of them will still receive the Messages. This is because both of them are sharing the same Apple ID and therefore have the Messages app setup to send and receive from both of their individual phone numbers. In the interest of a private conversation, my recommendation is that you just call the person due to the current setup of their iPhones.

Axel F.

It is recommended that they use different Apple IDs. Nevertheless there is another way to avoid SMS / iMessages being sent to both their iPhones.

This needs to be done at their end. (Especially on the iPhone who has enabled "Forward Text Messages") Read below

1. Set up text message forwarding
   1. On their iPhone, go to Settings > Messages > Text Message Forwarding.*
   2. 
   3. Choose which devices can send and receive text messages from your iPhone.

Disable the other iPhone on this screen.

*Note: this needs to be done on Bothe devices if they both have activated Text Message Forwarding.

Yes it does because the results are dependent upon the receiver's settings (Your Son and Daughter in law's iPhones) and has no bearing on the sender's type of phone.

Axel F.