By lead lines you mean walking with a rider on? They are 4 and 5 and will be taking lessons this summer. Sarah a funny little girl. She's horse crazy. She tells me she wants a little horse ever since she saw Pony. Thanks a lot lol. My instructor says she is a "natural" whatever that means. I was thinking maybe for Christmas but I didn't know which would be easier and safer.
Honestly... 4 and 5 is way too young, and I wouldn't do more than leadline until they are a lot older. You can do a lot of that, they really enjoy it. No point in lessons even, you can tell them enough about hanging on. They don't have the strength or attention span to learn to properly communicate with the horse.
8 or 9 if they are still interested, I'd start them on lessons. 10 and 12, maybe their own horse if they are really interested in keeping up with it.
That's my take.