Best buffet for open house birthday party?

In addition to all these suggestions, I often roast a whole beef tenderloin and slice it thinly for little DIY sandwiches with tiny rolls or party rye, pumpernickel, etc. Also put out a good mayo and Dijon. People inhale it.
Peopl

I like boiled shrimp with cocktail sauce at a buffet. Also the cold cut plate with sliced cheeses with the mini rye breads and a good mustard. A bit mundane, but well liked by all.
One salad I like now is a mixed green salad with goat cheese, walnuts and for a the season; pomegranate seeds. With a lemon vinaigrette on the side.
Maybe some sliced radishes for even more color.
Maybe put the cheese on the side for the vegans.

What about bagels & cream cheese spread, lox, cookies, potato pancakes, etc.?

Keep it simple. You don't want baby to meltdown from all the excitement and the (to him) strangers coming through, because then it becomes stressful for everyone. He certainly won't know the difference between an elaborate buffet and a simple one and guests also aren't expecting an elaborate spread.

My little guy also has a bday during that week, and though he is a little older than yours, we did several of those between-holiday open houses. We always went with a generous, but not heavy, finger-food spread - followed of course by cake. I think people at this time of year actually appreciate lighter fare, and most people certainly understand that when you have a baby or toddler crawling/running around, time doesn't really permit hosting a complex or involved production. While I think the idea of a soup bar is really nice, why not go with one soup that people can customize with add-ons, like a lime-chicken tortilla soup. But honestly, if it were me (and it was!): I'd stick with a couple nice cheeses, a salume platter, good breads and crackers, grapes and clementines, mulled wine and bubbly water. Done - with minimal prep and cleanup

Here's another idea and one that can be put together ahead of time. I like to do a buffet this time of year with recipes that constitute a Provencal Christmas Buffet. The menu starts with this yummy Cream of Garlic Soup http://food52.com/recipes/3268-cream-of-garlic-soup and continues with several savory tarts and pizzas. The dessert table consists of one spectacular dessert along with dried fruits and nuts. If you're interested in these recipes, message me from my page here and I'll be glad to share.

Ah, the birthday open house, I've lived and learned! My best advice is to keep things as simple as possible. No one will be expecting a big spread, especially if you start at 1:00 pm. I think your soup/salad/bread idea is lovely, or you could just go with some snacks / hors d'oeuvres since most people won't be expecting a meal or anything substantive. And from past experience, I think 2 hours is about right for an open house -- much more than that and you'll risk exhaustion. More than anything, have fun and worry less about the food than having an enjoyable, memorable celebration!