dived bali for 6 weeks in nov/dec 06. here my thoughts. menjangan has very nice dive sites but is far away from the rest and also very remote. ie almost no activities outside of resort.
tulamben is the only place which consist of a small town. on top of that, i personally found bali diving at its best in tulamben. not the wreck itself, marine life around is much more interesting. but i did much more dives in front of the tt coral garden (tauch terminal) and the drop off. about 7 miles outside of tulamben is scuba seraya. a place you just need to see (muck diving, boxer crabs, harlekin shrimps, coleman shrimps and zebra crab)
also a must are the dives around nusa penida for mantas and mola mola. check it out, this is easely reached from the south. the dive sites on the north coast involve all a long drive. so it is adviseable to stay in the area. travelling with your mom, i think you have more possibilities staying in a small town rather then in a remote resort. i suggest, you stay in tulamben (tauch terminal) or more up scale at scuba seraya (but this is remote again) staying in tulamben you have the freedom of eating in different places for a change.
if you two look for a remote place just outside of tulamben do a google on alam batu. new resort. i liked it quite a bit but bear in mind, for the good diving you go by boat to the tulamben area.
for diving in menjangan just have it arranged for you, with whoever you dive or just transfer for lets say 3 days. when diving secret bay, don't forget your 5mm wet suit. the water is always cold.