I am not sure when she last met them. However, I can tell you when she last heard from them, talked to them.
Earlier hours in the night, Wills had called Diana after his concern that Eton was wanting to do a photoshoot with him, but not with Harry. He feared that it will outshine Harry. Harry, meanwhile, last heard of his mother when he was asked what he would like for his birthday. He had said he wanted a PlayStation, and Princess Diana, after hearing so, made sure that someone was able to buy it.
It is not accurate when was the last time Diana had talked to Charles. She might have called him after Wills had called her — they were already on civil terms these days, in fact described as their friendliest time of their lifetime. But before she had left Britain, it is rather believable that she has talked to Charles about their sons. That's about it.
The Real Diana by Lady Colin Campbell