Get the riders and a bike or two into schools, offer some free tickets out to the kids. It's not like the tracks have not to the space to fill.
I'm not sure of attendances nowadays up and down the country but surely if each promotion pick out 3 or 4 schools each week and gave them even 200 tickets between them to include a few adults to chaperone them, the increase in Programme sales, food sales and maybe a souvenier or 2 would help. If for every 50 tickets they give away, it peaks the interest of a kid who persuades a parent to bring them along in future, it has to be worth it.
There has to be a "cub" or an asset, even a former rider who could be at the track and let the kids sit on a bike and help sell speedway..