what do i do to protect her... HELP

so. i have a 2 year old daughter and an 8 month old daughter. and a son on the way. 6months pregnant. to a scumbag sociopath. manipulituve lying cheat of a boy. who has walked away on my kids. to go back to his ex... who he also has 3 kids with. so hes playing no.1 dad to those 3 kids. while its like mine dont even exist anymore. he hasnt seen them in 3 months now. and suddenly his facebook and whats app profile pics are of there 3 kids. which he also never would have done before now. but the thing is. my 2 year old hasnt forgotten him and is forever asking for him. i was hoping in time that would pass ans she will forget. but this is where it has recently become complicated. i have a son to a previous relationship. who is the same age and in the same school as there son. so i see her and there kids at the school everyday. recently her boys of 6 and 7 yrs old. are calking my daughtrr ans coming over to play with her talking about daddy and asking when she aill see daddy etc. its putting me on the spot as this isnt my doing. but it also isnt making him do anything about it either. so for me if he isnt gonna bother id rather my daughter forgot them all. but how if they keep allowing this. what cab i do to protect her from this.