If you're looking for a milder-flavored chard, try "Perpetual". I find it to be more spinachy and less beety, when compared to any of the colorful mixes. It's also resistant to bolting (hence the name), which is a nice bonus.
For kale, grow it in cold weather - light frost breaks down some of the starches into sugars. It can get quite bitter in the heat. Arlene, you're in the same zone as me, so you can probably grow kale as a winter crop, though Rita will need to aim for spring or fall unless it can be protected from the worst of the freezes. In fact, in mild years, even chard will stand a zone 8 winter for me, though I don't think the flavor benefits from frost. If you're using the kale raw, just pick the young, tender leaves, and they will also be milder. I don't find a lot of flavor difference in young versus old leaves of chard, though I still prefer the younger ones in salads due to their texture.
Either one will taste milder cooked. Throw a handful of leaves into your minestrone instead of spinach, and you'll hardly notice the difference. Eating a bowlfull raw as salad may be healthy, but it's going to have the strongest flavor because there's nothing to mask the greens.
