My advice to you, after having a party with 25 guests, is that the less you have to worry about during the party, the better. So no games or favors (she's only 1, so I don't think anyone expects it), but maybe set out a basket of age-appropriate toys for your guests' kids so they have something you approve of to keep them busy. For the older kids, maybe borrow a couple toys or board games from a friend?
Also, delegate responsibilities to your most trusted guests. Have someone be responsible for serving and refilling beverages, for cutting and serving the cake, for matching gifts with cards and getting rid of wrapping paper after they've been unwrapped, etc. Also, give someone the job of changing diapers and/or changing a dirty outfit (and leave a pre-approved outfit on the changing table!) so you won't have to leave the party.
Most likely, your daughter will be slightly overwhelmed by the activity in her house and will not easily leave your side. Getting help from others and leaving you available will be the best way to take care of her so that you can still enjoy her day. Plus, you don't want to miss her opening her first presents or eating her first cake because you're trying to be a hostess at the same time!
As for a theme, I used a variation on the theme my friends used for my baby shower, which is also the theme for my son's nursery. His bedding and decoration are stars, so I found a cute 1st birthday banner with stars and hung stars from the walls and light fixture over the dining room table. I used the same colors from his bedroom, and I also got lots of balloons - a big hit with the one-year olds! Go to your local party store, they have cute ideas. But don't go crazy with decorations, just put them where you know pictures will be taken, like behind the spot you'll open presents and behind the high chair where the cake will be tasted.
And, because a party for 50 guests can get pricey, I bought half of the plates, cups and napkins to match the theme and the other half in coordinating solid colors. Then I alternated the plates with the decoration and the solid colored plates (cups, napkins) and saved a lot of money!
This is a big day - congratulations on making it through the first year!