I dont want to sound like a smart ass now, but you can actually train forearms a little more than you can other bodyparts.
If you do deadlifts, rows, curls, chins and any other back or bicep exercise, they get taxed hard from that alone.
On back day, do a few wrist curls and maybe some reverse wrist curls or if that hurts your wrists, do reverse curls.
i used to train forearms once a week and that did not do much for them, i now traint hem twice a week and they have started to really beef up on me.
Do two other forearm exercises on another day and they should soon thicken up.