I ran an intern program for a year as a software development leader at a previous job. About 25 interns ran through the program that I created. Each intern was paired with a full-time mid to senior-level developer. Out of the interns, only about 5 were decent and required very little handholding while the others required quite a bit of work to answer their questions and keep them on task. Of those 5 decent interns, I only offered full-time employment offers to 2 of them. I believe we paid $16-$20 per hour (the executive team set that rate) and most worked around 10-20 hours per week while a few were able to work upwards of 30-35 hours.
I can only speak for my experience, but I couldn't make it work financially and we would have gotten a lot more done had we used those funds to pay for 1 or more nearshore or offshore resources. It created a fun environment and I think everyone learned and grew from the experience, but from a productivity and financial point of view, I don't think it was worth it.
So based on your experience with Eastern European devs, I think your money would be best spent on hiring 1 or more additional devs from Eastern Europe.