Total Addressable (at least in the US) can be built based on market data sets, both free and for fee. For early child care centers, Market Data Retrieval and NCES (publicly available data set) are two recommended sources.
For Serviceable, that depends on a) your own business / operating model and b) the current and projected penetration of comparable competitors in market. For the former, consider things like technology usage and implementation capabilities. For the latter, if you have identified clear competitors, it is unlikely that you will have access to accurate info. So, if you can estimate these numbers and reverse-engineer, that's a good starting point. This would also be a great time to consider how your SAAS product is differentiated and will give you a chance to think through different scenario models for customer "switching" (from an existing competitor product to your own).