D.H.
For your daughter, offer a really non expensive, but nice gift....something she would like......maybe stickers to start out with......make a sheet (chart) and put them on there and show the days she goes etc......
For you son......start writing down the times he pees.....you can do this with your daughter too.......See if there is a pattern.....if so, then you will know when to take him............you can also take him, run the water in the sink, or even dribble a little on him.......a lot of times that works to make them pee......You can also buy a bulls eye to put in the toilet for him to aim at or even a piece of cereal.......making it fun for him.
Charts are nice, some kids really like them. Start out small with the prizes you give out for the results you get and then make it a little nicer once they get the idea....also, talk about being a big girl and boy.....how they will start school soon and how big they will be........how impressive....ect.......all praise is good......
Good luck and unfortunately, you may have a time schedule, but kids don't ......some learn faster than others and some are not ready when you think they should be........so you may want to have them trained by 3.5, but they may have a different idea, either way, be consistent and confident.