Can I have a small birthday party for my kid here at an available table without reservations? By this I mean can I bring in a few balloons, and presents and have them at the table with us while we eat and he plays?

You will need to verify this with the location of your choice as reservations are their priority. I've seen families turned away at the door for wanting to do what you're suggesting.

Ask the location

Yes. but get ther early they tend to get pretty busy

Yep

Yes

Yes of course you can

Yes you can