You should be fine with an 18 month old ... you might try for "off-peak" dining times and hopefully you aren't going at a REALLY busy time of the year. No matter what, just be prepared for whatever reaction your chid has. If it's enjoyment, snap tons of pics! If it's fear, don't force it. I don't think the Disney characters would ever force themselves on a child, just be prepared that you may need to leave the restaurant to calm your child down.
We took my nephew to Crystal Palace for dinner when he was just shy of 6 months old ... he totally LOVED the characters - he was all smiles and let them hug him and stuff. Totally worth it for us!
He was very used to Pooh and Tigger though as my sister has huge, talking Pooh and Tigger stuffed animals. Also we knew to let the baby see the characters coming head on, no surprises. (When my sister and I were teenages, we had a horrible experience at our local mall when a non-Disney character snuck up on my cousin who was about 3 years old. He was terrified of characters for the longest time. His first trip to Disney, he hid behind one of us whenever a character was in sight! Second trip, he actually agreed to pose with Mickey and with TweedleDee and TweedleDum - just not alone. Different kids, different reactions.
We're heading back this summer - my nephew will be 14 months old - and going back to CP and trying Garden Grill.