A dwarf lion would be fine in a 30, and as long as you added fish with several weeks in between each addition, it would be fine to have a couple of other small tankmates, the biofiltration will adjust itself. If you want the eel, I would go with the 55 and have just the lion and the moray, that would be fine. Triggers do not go well with lions, they are very aggressive and will nip at any fish with flowing fins. Also, if you get the eel, you will not be able to keep shrimp, as the eel will likely eat them. Zebras are not a threat to fish, but they will eat crustaceans. Dwarf lions are not much of a threat to fish either, as their mouths are not as large as their larger cousins, although it it still capable of eating some pretty large things. Rather than trying to lock the lid, get your child involved in the tank, and teach them that playing with it can kill the inhabitants, so it is important for him/her not to play in it. If you let them help you do things with the tank when you work on it, they will be less likely to play with it out of curiosity. HTH