Is your idea of conversion to a sloop just removing the inner forestay? I'd say no, but if you're worried about mast support you can install an actual baby stay. Which is a small forward stay, not meant to support a sail, but meant to help bend the mast.
Be aware that the geometry of a cutter rig will be a little different from a sloop... the mast may be placed further aft, for instance, and the boom may be shorter. Many cutter rigs will have a sprit. The intent of such a rig is to spread the sail plan a little more laterally... and thus easier to handle with a few extra options.. Things cruisers enjoy.
If your daughter's having problems handling the headsail because of the inner forestay, there's a technique and a remedy. The technique is to allow the headsail to slide against staysail, controlling the sheet rather than casting it off, until the forward part of the sail finds its way through the gap between the stays. The remedy, when the technique is balking, is to install a cringle on the foot of the headsail about 50% back from the luff. A control line is tied to the cringle and run to a small block at the stem head, then back to the cockpit.... this allows the crew to pull the front half of the sail up into the gap and bring the clew/sheet past the inner forestay. Good luck.