Do not give them an exact figure for what you'll budget for travel each year. What if you want to take a trip somewhere else as a family?

I think it's fair to say something like,
"Phil and Martha, thank for offering to pay half of the airfare for the kids this summer. We are really sorry that we were unclear in January, but we said that we couldn't afford to fly out ourselves because we don't have the budget for airfare this summer. We had assumed you would continue to pay for the kids' tickets as you have so generously done in the past, and we should not have made that assumption. We do not have budget this year to pay for half of the kids' airfare, and we are sorry for not being more clear about it and thank you for the offer to pay half!

We would REALLY love for you to spend time with us and the kids this summer, and we would love for you to come visit here. The kids would enjoy having you here very much. After this year, maybe we could consider a tradition of rotating every year, so we would budget to save for airfare to fly to you every other year, and you could come to us every other year. We love spending time together and especially love that you get to spend time with the kids."