Hello. Was hoping if someone could assist me with some UK visitor visa questions please

My parents currently live in the US on green card and are planning to apply for a UK visitor visa to visit us in the UK.
One of the questions on the application form is around travel history. We''ve answered that best we could based on passport stamps. However, in 2015 they travelled several times between their home country and another one they were residents of at the time. We can't find stamps for those visits as they entered based on their residency card, and we can't remember the exact dates. Would it be too much of a problem if we don't enter those trips in travel history?
Also, do my parents need to write a letter in addition to the sponsor letter I'm writing?
Thanks
