regardless of how you choose to handle this situation, I commend you for asking for guidance on how/if/when to approach someone with a disability!

IMO I would go with saying something akin to "Hi! I look forward to seeing you and the baby as we cross paths on the trail. My that is a cute little gir/boy - how old is he?" Then you could perhaps say somehting to ease into the conversation like "Gee, I need to sit down for a moment due to a disability - care to join me and get to know each other a bit better instead of just staying strangers who walk on the same trail each day?" blah blah blah.

You could ease into the convo via the kid (we all love to hear a compliment about our cute, well behaved children) and then ease into having a disability but "looking so good". She will either open up or not, and that will let you know how comfortable she may or may not be with "sharing" amongst people she meets.