Best location for kids? Hmmm. Most hotels are in and around the downtown core, close to most things, but of course, the downtown is built for businesspeople, not for children. If you stay in Old Montreal, you'll be close to the river and the Science Centre and IMAX theatre, which will keep the kids interested for an afternoon, plus you'll be in the historic part of the city, which is very pretty. It's not in the centre of things, however -- there are enough restaurants and tourist shops, but not much else. It depends on your tastes -- if you want old, beautiful and romantic, try Old Montreal; if you want modern buildings, shopping malls, food courts, and so on, downtown is better.
Montreal is a city that is best seen on foot and by métro or bus, so be prepared to walk a lot! The problem in March is that it won't be warm enough, so there won't be much stuff going on in parks and you'll be doing things indoors. Try this post for a few ideas...
I don't think that there is any skating this year at Beaver Lake, I think the site is undergoing renovations. Given the mild winter we had so far, not sure that you would be allowed to skate there in any event.
The refrigirated ice skating rink is opened at Beaver Lake on Mont-Royal. There is another one in Old Montreal. You can rent skates at both these places.
Park Lafontaine also has a very nice skating rink. The quality of the ice depends on the weather.