The OP has asked for some ideas on what besides boarding could bring extra money to offset the cost of the new barn. I can suggest a couple ideas which may or may not appeal to her. One would be to breed a horse or pony to be sold when appropriate. That would take another stall and would have up front costs for future payout. But it would give her children additional experience in horses that just care for adult animals doesn’t.
If more horses are not desired, are there any high value farm animals they could raise to sell. Some types of poultry can fetch high prices if an upscale buyers market is available. I know some folks who raise particular breeds of turkeys which go to families which want only these for their Thanksgiving or Christmas table. One lady keeps milk goats and provides the milk to someone who makes high end soap. Goats (or sheep) have a fairly short lactation period so milking is not a year round chore. Maybe a talk to the folks at ag extension in your area could provide some ideas.