What's the fastest way to get from Laolongtou to Beidaide?

When the visit to the Wall is over, you may take bus no. 25 in front of the scenic area to Shanhaiguan Railway Station. There are several trains leaving for Beidaihe daily, and you will arrive in Beidaihe after 30-40 minutes' ride. The ticket price for a seat on the train costs about CNY10.
Upon arrival, you can take bus 22 to the Quzhao bus stop and the scenic area is very close.
If you go to Beidaihe from the Wall via taxi, the taxi ride should cost CNY100-120 in usual time.

Ok, thanks very much for that answer. I looked at the train times, but saw that there aren't any that area really convenient for us. Do you now which buses (only buses) we can take from Old Dragon's Head to Beidaihe? Also, is it possible to walk on the wall from Shanhaiguan to Old Dragon's Head? I read that it's about 5km... Would that be on top of the wall, or just along some streets where the wall used to be? How long might this walk take? Thanks again!

In this case, you may take tourist bus no. 501 from Old Dragon's Head to Beidaihe. But the bus only departs at 14:40, 15:10, and 15:40. If it is still not very convenient, maybe you should take a cab to go there directly and I have written the reference price in above.
Shanhaiguan and Old Dragon's Head is not connected. You need to follow the street to get there. But 5km is really a long way if you go on foot. It is recommended you take bus no. 25. It should take 20 minutes.

Thanks again, Nichol! Those bus times are better than the train times, but we still may take a cab as you suggested.