we transitioned DD to a big girl bed with relative ease. first we transformed her cot to the toddler bed arrangement and then when we found out we were expecting went to Ikea and got a toddler bed that is extendable to a full size single. we both fit on there easily as i have been sleeping beside her the last 2 nights as she has been unwell and needing extra cuddles...and we fit comfy even with my 30 week pregnant belly! (we also just went straight to the single size, as opposed to toddler size first as we wanted to buy a better quality, long term mattress for her, rather than the toddler ones that have bits and bobs etc).

if i were in your position i would do a transition to a big boy bed. each kid though is going to react differently but in our case DD loved it (she was roughly 16ish months when we converted her cot ITH).

goodluck with it all! i am sure you will find a happy arrangement for your babies xx