Long road trip with a 2 year old

[lunarophelia]

In three short weeks we will be loading up our car with our 2 year old son, 50 lb dog, and very vocal siamese cat and leaving Anchorage for Seattle. From there we will be heading to Salisbury, Maryland. Any survival tips? Specifically pointer on how to keep the aforementioned toddler from peeing all over his car seat? I don't want to put him back in diapers because he will riot (NO MAMA! IS FOR TINY BABIES!) but I can't afford to spend $10 or more on cloth training pants. He has two friends in pull-ups and when he sees the pull ups he calls them diapers, so even going the disposable route would be a tough sale.

Also, any advice on preparing a toddler for a move?