The simple fact is that your sons are entitled to basic information about the Trust since they are the only beneficiaries
You can find out more here
It is clear that they did NOT appoint solicitors to deal with the Probate - so it is unlikely there are solicitors dealing with the Estate
At present since your youngest child is still under 18 YOU as his representative are entitled to this information on his behalf.
Your sons are also entitled to ask the Trustees to advance them some monies - a net estate of £263,000 suggests that there should be funds available even if only from the assumed rental.
The executors are personally liable to your sons for the proper administration of the Estate and the Trust.
I woudl suggest that a clam and formal letter is sent to the Executors reminding them of their legal liability and requesting the basic information would be the way forward
Please ask if you need further details