Your ILRs will remain valid as long as you and your daughter continue to live in the UK. If either of you leave for more than 2 years, your ILR will become invalid and if you want to move back to the UK, you will have to start over from scratch with 5 years of visas again (but as your daughter is over 18, she would not qualify as a dependant anymore and would have to qualify for visa in her own right, such as a work or student visa, but a student visa does not lead to ILR).
So, your best option, in order to give you both the freedom to live either in the UK or US in the future, is to apply for UK citizenship before she leaves the UK, as once you have that, you won't need to worry about ILR expiring in the future.
Citizenship is easy - much easier than qualifying for the spousal/FLR/ILR visas
. You barely have to provide any documents at all.
If you are married to a UK citizen, you just have to show:
- you have passed the Life in the UK test
- you hold ILR
- you have lived in the UK for at least 3 years (you must have been physically in the UK on the date exactly 3 years prior to your citizenship application date
- you have not been outside the UK for more than 270 days in the last 3 years, and no more than 90 of those days can have been in the final 12 months before applying (the day you left and returned to the UK on each trip is not counted).
If you are not married to a UK citizen, you must:
- have passed the Life in the UK test
- have held ILR for at least 12 months
- you have lived in the UK for at least 5 years (you must have been physically in the UK on the date exactly 5 years prior to your citizenship application date
- you have not been outside the UK for more than 450 days in the last 5 years, and no more than 90 of those days can have been in the final 12 months before applying (the day you left and returned to the UK on each trip is not counted).
I think your daughter may be able to apply for citizenship at the same time as you, but as she is over 18, I'm not completely sure... so you'd have to check that.
Because of the residency requirement, I think she would need to get her UK citizenship before going to the US for college, because otherwise she might not meet the 'days outside the UK' requirement if applying at a later date.