there are always family activites in SA
alamo of course, but do the whole mission trail, on tour bus or on your own, one of the missions (still an active parish church) has all the old buildings open and working
buckhorn saloon (I hate it but grandkids love it)
river walk must take the boat ride
the mercado, eat at Ma Tierra
the Cathedral, spectacular, try to get to Mass there if possible
also shrine of St. Theresa of the Little flower, an easy busride from downtown, another must see
Lourdes Grotto at Oblate School of Theology, there is a plenary indulgence this year (under the usual conditions) for a pilgrimage to Lourdes or a grotto where this apparition is venerated
don’t know if sea world and fiesta texas amusement parks are open, check the internet
off the riverwalk, the Mall has an IMAX theater, it always has the Alamo movie, also other selections.
the internet site has all the info, and discount tickets for a lot of stuff
you will like shopping in La Villita, kids probably won’t be thrilled but they will enjoy riding the trolley all around downtown.