Do you think they are? I think yes. Here's why.
When we first see Goten transform, his appearance in the form is very similar to that of Gohan and Goku's when they exited the Room of Spirit and Time... no aura, round eyes, and a lighter shade of yellow for their hair color (in the anime at least). None of these features were ever shown on a non-mastered Super Saiyan.
On top of this, they have the same aura as a Full Power Super Saiyan.
Their strength may also support them being Full-Power Super Saiyans. Not saying this is evidence, but when we first see FPSS we see how strong Goku and Gohan are compared to Trunks and Vegeta, despite having less time to train. Goten, based on his sparring with Gohan and guidebook info, is very similar to Gohan in power. So, perhaps the reason for them being so strong is because they're FPSS?
And finally, we see Trunks has the ability to suppress his Ki in Super Saiyan. So are they FPSS, or no?
Is there any supplementary (guidebooks) info?