Just plainly explain to them na nakapagusap na kayong magasawa and you have decided to combine the celebration of your baby. You may tell them na nakapagcoordinate na din kayo sa suppliers and you think it's the best idea to combine the party this time. I think pag naipaliwanag nyo naman in a nice way, they will not take it against you. Siguro sasama ang loob pero hindi naman magagalit since kayo naman ang parents ng may birthday.
Magbasa pa