My dotter had the SAME problem when she started on cereal at 6 months. She use to not poo for 10 - 12 days. Imagine our horror. They hospitalised her but couldn't find anything wrong also. We gave her everything from fruits to fruit juice to glucose water. NOTHING worked. We didn't want to rely on lactulose and suppositories too. We just didn't understand why she constipated so bad. She constipated for another 6 months and most times there's blood in her hard stool too.
Then when she turned 1 year old, I gave her Yakult. By chance only. Wah..she poo soft lahhh. When she turned 14 months, I gave her 3 scoops of mango/peach yoghurt everyday. Even better. Her bowel has improved tremendously eversince.
But of course results vary with different child. On top of giving your child more liquid everyday, I think you gotta find that particular food/drink that makes your child poop.
All the best!