March into April is actually a pretty good time to visit Scotland. You can avoid most of the tourist hordes, and flowers (daffodils and tulips, mainly) will be starting to bloom.
Your itinerary looks fairly doable for the first couple of days. Stirling Castle is about 1 1/2 hours from Edinburgh, depending on where you're staying in Edinburgh. I think that the castle opens about 9:30 a.m., so you should aim to get there as close to opening time as possible. The Trossachs are an easy drive from Stirling. The A84 will get you close. Take the A81 south from Callander to Aberfoyle, then the A821 north through Brig O' Turk back to the A84. If you have time, you may want to take a one hour cruise aboard the Lady of the Lake on Loch Katrine. If your schedule doesn't provide enough time for a boat trip, there's a good walking path along the north shore of the loch. Loch Katrine to Oban should take you about 90 minutes via Tyndrum.
If you're planning to visit Glencoe on your way to Mallaig, allow yourselves at least five hours for the trip. Probably more, if you plan to stop along the way. They are good roads along the way, but you're going to want to stop to take in the scenery more than once. Don't forget to make your ferry reservation for Mallaig to Armadale, and to make reservation for accommodation on Skye.
March 30th. is where I think that you may need to reconsider your itinerary. As you mentioned that you really want to visit Skye, you are not giving yourselves enough time to give Eilean a' Cheo (the "Island of Mist") justice, especially if you plan to visit Eilean Donan and head for Inverness later in that day. You may want to spend an extra night on Skye, and head to Inverness by way of Eilean Donan and Urquhart Castle on the morning of the 31st. If you get in to Inverness early enough, you could visit Culloden and the Clava Cairns before dark. If not, you could visit them on the morning of April 1st., on your way to Nairn and Elgin. There are several very nice side trips from Elgin, including dolphin watches at Spey Bay.
On April 2nd., driving back to Edinburgh via Peterhead and Aberdeen will be very tiring. Unless you have a specific reason to go by way of Aberdeen (Dunnottar Castle, perhaps?), you'd be better off choosing an alternate route. You could conceivably save Elgin for this day, and take the A96 straight down to Aberdeen, avoiding the lengthy journey around the coast. You'll probably want to stay near EDI if you have an early flight on April 3rd. If you decide to head directly back to Edinburgh from Inverness on April 2nd., there are several good side trips that you can make off of the A9.
As acraven pointed out, you'll need to prepare for any kind of weather. You could encounter temperatures in the 50s and 60s, or you could encounter snow. Keep a close watch on the weather forecast in places where you're going, and remember that they're right at least 50% of the time.
Best wishes for your travels, and happy holidays!
p.s.: Your daughter will be of legal drinking age when you arrive in Scotland, so she can have a pint or a wee nip in the pubs. If you want to see some live traditional music sessions at Sandy Bell's (Edinburgh), she'll have no trouble staying for the evening.