Just a different perspective....when I turned 50, 2 years ago, my husband came down with a terrible flu, so did my 8 year old, and they slept on the couch together for 3 days, medicated with flu meds, and I had to cancel our nice dinner reservations and ended up going to a new friend's surprise birthday party. It's was a bummer and I wouldn't recommend that route to others. But I had fun, even though it wasn't all about me.
Your family really likes to 'overdo' birthday's and they like to golf a lot, IMHO. You've already celebrated once, so when does it end? Is your mom particularly close to her brother/ your uncle?
A 2 hour drive with 2 little ones is a bit long, especially alone. Do you have DVD player in your car? Will they nap? It's always nice to see family when you can, especially when your a single mom. You're kids will benefit tremendously from being exposed to good family members.
But, I don't think you should intentionally left out. I think your being a little too sensitive about the plans and how they unfolded. Maybe they got a smoking deal on golf fees for booking back to back sessions. Who knows. If you go, go for the fun of it, not because you feel bummed about being excluded.