Is it worth talking to her and understanding why she wants to start seeing them again?
She's an intelligent wee girl - can you also explore gently the reasons why she stopped seeing them (because they were bad mouthing her mummy and it was upsetting both her and you) - and what might have changed since then.
Not sure your ex-p will be much help - I seem to recall he wasn't last time when this was happening.
But I do think you shouldn't be seen be putting an obstacle in the way of her seeing them. If she genuinely wants to, let her - but monitor it closely. She might just be wanting to 'cos her new friends at school are talking about grandparents. The reality might have got rose tinted in her memories and the reality will bring her back to earth!