Traveling with two small kids is a whole lot harder than traveling with one. If you are planning on long international flights, I highly recommend buying a seat even for children under 2. You are going to need more miles to buy 4 tickets, and I would plan on traveling in coach until they are both at least 6. Even the best behaved kid can have a meltdown when they're tired, their routine is disrupted, and they are expected to sleep in a strange place. Excited kids speak in loud shrill voices even when you ask them 100 times to speak quietly. I think you'll find yourself holding off on those international trips a lot longer than you think once the reality of 2 kids under 3 hits you (mine are 4 and 6, and I'm still afraid to take them on a 10 hour flight).
So on one hand you are going to need more miles for international family vacations, but if you are accruing them fast and you have a while until you use them, you might as well burn some. But if you are short on cash now, maybe you should consider waiting a little longer before having another kid. Two are almost twice as expensive as one.