Due to the high worm infestation rate in developing countries like Nigeria, routine deworming is recommended for children, 12 months upwards and should be repeated at six (6) months intervals.
You don't have to wait for the symptoms which are numerous from failure to gain weight, to low blood level, to anal itching to passing out the worm in faeces etc before you deworm. There are different types of deworming drugs and they can be bought without a doctors prescription.
Albendazole is a good choice. The dosage should be given to you by the pharmacist based on your child's age (200mg /400mg; 200mg for children below 2 years and 400mg for those above 2 years).