Estranged family

My partners ex did a runner with kids his youngest is now 17 has got in touch wanting meet him and our 5 year old but it's conditional that she wants me her brother which I think is great but I am not allowed to be there and she wants get know our son but I'm not allowed to come to any meetings going forward and then hits out with once he knows who I am ill take him on days out alone. I'm pissed because I'm expected to allow a stranger around my child amd say yes no problem and say yes to taking him out my partner has asked how I feel I did say I'm not comfortable with this at all and in my gut I feel something is off. My partner is meeting her alone tomorrow and is going explain we would love her to get know S but she needs to accept I'm his mum and if she can't be a grown up and accept me she can't be around S for now then she said OK we can meet at park but L has to stay in car.