I am in need of a long rear mudguard for mtb pulling a child trailer.

No mudguard mounts so needs to be seatpost mounted, crudguard type but longer.
Any suggestions?

decathlon do some full length clip on guards for a mtb.
have a look on their website.

Having it long off the back will not help with the child trailer as much of it is below the height of a clip on mudguard. Just use a normal full length rear one and some p clips to attach to the the seat stays. Add a mud flap to completely protect the little one.

If you’ve previously ridden without one, thereby subjecting those behind you to a spray of road grime, shouldn’t you force your kid to get used to that too?