If your daughter wants a 3DS to play portable games (with nothing specific in mind) then waiting for a Switch instead of getting a 3DS is probably worthwhile.
If, however, your daughter wants a 3DS for specific titles, you're still better off getting her the 3DS. For example, if all of her friends are currently playing and trading on Pokemon Sun/Moon, getting a Switch will basically have her feeling left out.
In her case it's not for a specific title, so she was excited to learn about the Switch and is waiting for it. For whatever reason I don't see many kids around here playing on 3DS, so peers aren't a factor. (Most gaming with friends happens on PC/Mac Minecraft; I run a private server for them.)
>> delayed gratification is not always a bad thing.
It's not so much about delayed gratification -- when you're talking about a specific game (especially one that is online) on a specific platform, it's more like no gratification at all.
If, however, your daughter wants a 3DS for specific titles, you're still better off getting her the 3DS. For example, if all of her friends are currently playing and trading on Pokemon Sun/Moon, getting a Switch will basically have her feeling left out.