At platform #3 of Toyohashi Station where Meitetsu Line trains depart, there is a ticket office to buy Meitetsu Line ticket.
First show the ticket to Toyohashi (such as Tokaido Main Line ticket to Toyohashi) and tell the clerk the destination and how many adults (12 or over) and Children (12 or under). In Japan, train fees differs depending on age. As far as your children are attending elementary school in your country you can declare your child as children. Pre-school children are free of charge in Meitetsu line. Then tell them to get ticket to HIGASHI-OKAZAKI station. Not OKAZAKI station. The fee is 550 yen for adults.
From Toyohashi Station only express, limited express and rapid limited express trains depart, to change the local train you must get to Ina to take a local train.
See my comment on Meitetsu Line guides.