I would be far more worried about Strongyles and Roundworms in weanlings than I would about Tapeworms.
If your mares are free of Tapes it is highly unlikely (virtually unknown) for a weanling to b affected.
Your whole herd would have to be riddled.
I would be looking at your whole worming "schedule" myself and doing something about encysted small strongyles and Roundworms, which will need a five day course of Fenbendazole to get really rid of them, and leave the Tapeworming til they are yearlings, or at least the next eight week rotation.
Again, I will warn, FENBENDAZOLE is NOT suitable to be used as a single dose wormer.......