This is the rules for our local pool:
Children under the age of 8 should be accompanied in the water by a responsible
adult of at least 16 years of age.
* One adult may accompany up to two children under 8 in the water.
* Children under the age of 5 should ideally be accompanied on a 1:1 basis in the
water, regardless of swimming ability.
I think they are pretty strict, but it does say "ideally" so I guess you could argue a case - especailly for twins! My husband used to take our two on his own when they were little and they are only a year apart, but we lived in another town then.
Look up the swimming policy and if it is biit vaugue, you could try your luck, I think two adults to three kids looks better than 1 adult to two kids.