Former Russell Group admissions tutor here, echoing @lanthanum and @MadridMadridMadrid . UCAS guidance suggests that teachers give PGs assuming that the wind is at the pupil’s back. The disparities between PGs and achieved grades last year suggest that this may have become exaggerated.
OTOH it is sadly well documented that some schools, particularly those that do not send many pupils to university, routinely under-predict. This is a grave disservice and should be stamped out.
You need to know what your school is doing to know how your DC should use their PGs. The ideal PG is lightly tinged with optimism, encouraging growth but recognising that there is no point encouraging offers that will not be achieved. In this case I would suggest 2 or 3 applications at PG standard, 1 or 2 a bit below it and 1 dead cert. In a highly competitive field that would be 2,2,1.
If the PGs are highly optimistic, 1,3,1 or 0,4,1 and try to get the school to see that the policy makes no sense.
If the PGs are pessimistic, 3,1,1 or 4,0,1 and launch a social campaign. This is holding your DC back and it is just wrong.