Hi
I jag mine with Ivomec , once a year - treats both internal & external parasites. At a year old I'd recommend investing in a slapshot & gun, unless you have somewhere you can barricade them in (weaners are easy enough to use a syringe, as they can be picked-up).
I them check at the 6-month mark to see if they need doing again - I would then use Flubenol powder - no problem them taking it in a jam sandwich (no butter, not homemade jam or bread & never been in your kitchen!). You can get a worm-count kit (Supplies for Smallholders on here does them), so you can see if they have a burden before you decide to treat them.