Since it's an English saddle, you can punch more holes in the stirrup leathers using a leather punch (providing they are leather) or buy a set of child-sized Wintec Webbers. Inexpensive and will raise the stirrups up to fit very small children (my lil one has been riding since she was 5).
Also, while a yardsale helmet seems like a great deal, you have no idea how old it is or how many times it's come into contact with the ground or other objects. Plastic deteriorates over time and it's structural integrity degrades every time it comes into contact with a hard object (even if it looks fine).
You can buy a new helmet for less than $30 and know that it will be structurally sound should your child ever end up falling.