Depends on which tablet the child consumes.
If the child is not able to swallow the tablet, he/she might choke, which is very very dangerous. If the child swallows it, if the table was meant for adult, consult a health-care provider immediately without delay.
If the question was for chlorine tablet - swallowing a chlorine tablet is very dangerous, event for an adult due to it's highly oxidizing property. Consult a doctor or poison control immediately, meanwhile, let the kid drink plenty of water. Induce vomiting if the child is above 3 years.