In order to make your little think that they need thicker diapers, start them off with cute pull-ups that hide well under the clothes that they already own. Tell them that the potty is having issues, and that the plumber is coming out in a few weeks to fix it, making it so that the only place they can relieve themselves is their pull-ups. After a few weeks of making them drink a lot each day, your little will need lots of changes, as their pull-ups do not hold that much, and after they start leaking, you suggest that they can either get their pull-ups changed twice as often or they can wear thicker diapers.
Naturally, all those in diaper training cannot stand leaking, so your little will choose to wear thicker diapers. It is important to make them realize that they need better protection, and not just because you say so. Having your little wear clothing that allows you access to their diapers at any time is important, so you should throw out all of their pants and replace them with short skirts and onesies. This gives your little the confidence boost they need to start wearing thicker protection, which will only make their bladder control grow weaker and weaker until they’re fully diaper dependent, as all good girls should be.