I will suggest Legoland then Sentosa. Mainly due to avoid too rush from legoland to changi.
This cross border traffic is hard to guess. Sometimes very smooth but sometimes can be very bad. Also, u do not need to bring the snack like lam chi Yuen together to legoland. Btw, try salted egg fish skin if u see it.
IF Family Arriving singapore but take/ checkin johor bahru residence in Malaysia ...
Not sure if my understanding is correct. If you plan to stay in hotel in Johor and visit SG every day, it is not convenience. Cross border everyday is expensive (in term of money and time). I will suggest only stay in Johor when you are in legoland/hello kitty land etc and stay in hotel in SG when you visit SG.