Keep it colorful, simple and SHORT! Babies that age will remember nothing of it, and it will all be a pleasure "of the moment." I would suggest a circus theme--it's easy and fun for babies. Buy clown theme paper plates and cups, and , if you bake, make a round cake with cupcakes cut in half for the ears, one half for the nose (red) and a triangle of cake (can buy pound cake to make this piece) for the hat. Decorate each part in different color frosting...white overall, red nose, blue hat, dots of colored frosting for eyes, dots on the hat...(be sure and stick on the half-cupcake ears!). Go to Google for "calliope music CDs" if your local library doesn't have this circus-type music, to play quietly in the background. .Or have a CD with "I Love A Parade" on it, and let the walkers "march" around the room, parade-style, following an adult. Some kind of circus or parade music is essential to the spirit of the party! Have red, blue, green, yellow pointy paper hats for each child. and put same color of crepe paper streamers (twisted) across the ceiling or hanging down over the cake table. Just make it colorful. Helium Balloons--red, green, blue, yellow tied to the table corners (Note: not for kids to take down ...they are dangerous until past age 5, due to possible ingestion if inflating). That's it for decorations and cake (and of course, if you don't want to bake, you can specify to the bakery that you want a "round, clown-face cake in primary colors".
Infants/early toddlers are all over the place, and the Moms will probably be holding them during the party, so you need not worry about "entertaining" them. Greet the little guests and their Moms or caretakers (hopefully everyone comes on time and the greeting will be over soon), hand them a party hat, then, when they are most all there, BRING OUT THE HONOREE, who has been hidden in another room with a helper...She/he will be amazed to walk in and see all the decorations AND all the little friends assembled! Get a photo of that surprised face! The party begins. Sit and chat for 15 minutes, letting the babies get settled, then announce refreshments: (Consult the baby's Mom for appropriate finger-sandwich fillings (plain bread and butter?) or other dry snacks for the babies, and you can serve the adults stuffed croissants, fruit and vegetable salads and rolls, or cheese and crackers and drinks if you want to include food. Depending on the time of day, just cake and drinks will be enough for all, though.. For the babies, you can serve plain spring water in one pitcher and lemonade in another, so they have a choice when having their cake. After refreshments, have a helper quickly pick up cups and paper plates and then announce GIFT-OPENING. Set your baby in the middle of the floor with an adult helper (Mom?) and go to it! You, the hostess, be sure and take photos! And remember, most babies are clueless at this age as to what is actually happening, but they do have fun at these parties!! Give each child a little paper bag containing a couple of treats as they leave--ask baby's Mom what would be appropriate to include (nothing that they could choke on!). Good luck ,and I think you will really enjoy giving your party!