Is it permissible for me and my two-and-a-half-year-old daughter to travel alone for Umrah, and for my husband to meet us at the airport in the Kingdom of Saudi Arabia? And would Umrah be accepted in this case?

If this 'Umrah is her first 'Umrah (i.e., 'Umrat al-Islam), then it is permissible for a woman to travel without a mahram if she is with a trustworthy group. However, if she has already performed 'Umrat al-Islam, then it is more prudent for her not to travel without a mahram, even though some scholars permit it. Meeting the husband at the airport does not suffice for a mahram when one is required. The acceptance of 'Umrah is solely with Allah.