K.H.
Hi
18 months is around the age when seperation anxiety sets in anyway , whether they have been with you full time since birth or if they have been left in daycare/with nannies etc , they still go through this. They have to learn that it is normal for you to leave them with other people and that you will come back. Also for your own sanity you need to have some time out aswell. You should join the gym and get them used to be with other people and kids without you there , after a few times going they will be fine & you will feel much better for it. My youngest is 16 months and has been going into the childcare at the gym since she was 12 weeks so no yours are not too young , she will also cry on the odd occasion but as soon as I have left she is fine , also the worst thing you can do when you drop them off is hang around for a while or keep going back to them , just take them in , tell them you will be back soon and go work out! once you have done it a few times you will love it.
Good luck
K.