I usually wait a few weeks but that is with mares that were rode & kept in shape throughout the pregnancy. I ride mine throughout until they get close & too big/grumpy, but at that point bareback is the only option and it's like doing the splits to ride them lol! If she's had a substantial amount of time off, I'd wait a bit longer.
I don't lunge mine (any of them really once they are broke) but I'd think having the mare on a line and the foal running loose could potentially be dangerous. I ride mine in the indoor at first with the foal along side. When riding the mare, keep in mind how much the foal can do as much or more so than the mare. Once they are halter broke and leading well, I pony them along side the dam all over the property. Closer to weaning time, the little one gets to hang out with a trusted older mare while I take mom away for a ride.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.