I've tested/played the game for over two years now, and I almost always have 6 kids in each generation. The key is to play the game often, and start attempting to make another baby as soon as possible after a baby is weaned. Baby boost can help, but persistence is the most important factor. If they argue, overrule them and keep at them until they try. If you have a lot of money, give them loads of baby boost - it uses up some of that excess money, and it does improve their chances of success.