Our dentist says that, at this age, the important thing is to set up a good, willing, twice-daily, teeth-brushing habit. So, if you've tried lots of different toothpastes, not to get into a fight over toothpaste but to go ahead and brush without any.
Obvioisly, avoid sugary foods, but also avoid sticky foods like dried fruit after meals.
Brush for as long as the child will tolerate, aiming for two minutes. Praise good brushing.
Tastes change with time, and, even if they don't, an older child may have better understanding and be more willing to co-operate.
We've been doing this with dd since she was about 3. Her teeth are absolutely fine, no decay. Now, age 8, she brushes with an electric toothbrush and no toothpaste, but a couple of times a week I brush fir her with a dab of toothpaste, and we will gradually increase.